Warjacks (usually shortened to just 'jacks, even in-universe) are the eponymous "warmachines" of Warmachine and the Iron Kingdoms setting. They are, essentially, multi-ton, steam-powered robots powered by magical brains called cortexes. They are the direct descendants of the Colossals (Titan-sized steam-powered robots) the peoples of the Iron Kingdoms used to fight back against the Orgoth invaders and then abandoned for being too impratical.
Technically speaking, however, they are only a small subset of the multi-ton, steam-powered robots, which are usually called steamjacks. Warjacks are specialized steamjacks built specifically for war, although some factions, notably the Protectorate of Menoth and some of the Mercenaries, have more than a few industrial robots (laborjacks) pressed into service with some minor modifications and one or more weapons of some kind bolted on, making them basically steampunk technicals.
An average Warjack is not particularly bright, unfortunately, so they need to be lead into battle by specialized battle mages called Warcasters or, lacking one, a "jack marshal" who has learned to direct a single warjack with signs and simple voice commands. Over time, the 'jack and the warcaster will bond, molding the 'jack to the warcaster's personality and making it easier to control. During this time a warjack will become smarter and developed personally quirks, ranging form being more aggressive to being friendly to children. Quite a few warcasters have their own personal warmachine with a name and some unique toys (and in-game bonuses when used together).
As it's often the case with military hardware, manufacturers prefer having several variants of a single chassis rather than dozens of different models of giant fighty robots. Game-wise, this means many warjacks of the same faction are aesthetically almost identical and share the same base stats, but have different armament and special rules according to their role.
Cygnaran light 'jacks are all based on either the pretty average Grenadier (though its two permutations the Minuteman and the Hunter are awesome in their own ways) or the stronger but less agile Charger (which is hilariously effective for 4 points), while their heavies, with three exceptions, are made from either the Ironclad or Centurion chassis. Typically, they're better at hitting things than other warjacks, and they hit decently hard, but don't leave 'em out in the open or else you're hosed.
Almost all of them have both a gun and a melee weapon, more often than not better at ranged combat (though through their kit they aren't slouches at melee either). Notable are the Defender, with one of the best guns in the game and a cortex-wrecking shock hammer, the Cyclone with its dual Gatling guns, the Hunter with its armor-piercing sniper cannon and the aforemontioned Grenadier, which was basically designed as a rapid fire field mortar able to dig its own foxholes and fuck your shit up with a giant
pickaxe mattock if you get too close. Cygnar is not without anything that can fight up close, for a cheap option, it use the Ironclad, which carries a big hammer and has a decent melee skill, and it has an ability with its hammer that can knock down infantry near it. The Hammersmith is widely regarded as a Khadoran 'jack painted blue for a reason; it's tough, has two big hammers and pounds things into the dirt. For a more expensive option, Cygnar can use the Stormclad, an electric sword wielding melee monster that can slice through enemy warjacks, and when it hits things, lightning bolts hit nearby units.
Then there's the Thunderhead, which is one of the most advanced heavy warjacks to date, made from a failed attempt at making an stealthy, electricity-powered warjack that ended up as something best described as a 'walking lightning storm' (hence its name) that fucks shit up by venting lightning into everything around it, shooting lightning everywhere, or punching people with million volt taser knuckle dusters that automatically hit after you bop 'em the first time. The unique parts to it also made it too expensive for mass production.
Cygnar's colossal, the Stormwall, is giant wall of dakka equipped with a pair of huge cannons, and pair of gatling guns that fire off so many bullets that they can produce multiple AOE templates that hurt enemies that end turns inside them, and has an interesting ability to release a thing called a lightning pod that can be used to aid with Cygnar's other electrical tricks. Oh, and if it punches something (rather then doing a sweep with its fists), it releases electricity to hit nearby units. As Cygnar loves to support its shooting, most players regard it as the most useful colossal.
A Cyclone. Might have the most dakka in Warmachine.
According to their fluff, the Khadorans can't produce mil-spec cortexes as quickly as the other nations (even the Protectorate of Menoth somehow manages to produce more than Khador, though given their statlines it's possible that the PoM are using second-rate cortices), so they can't make very many 'jacks. As such, they refuse to use sissy light warjacks; (almost) all of their warjacks are of the heavy class, and the only 'light' warjack the have is a warcaster's personal attachment (also the only arc node they have). Khadorans also don't have much appreciation for aesthetics; all of their warjacks are big, blocky hulks with three-fingered hands, stumpy legs, inhuman helmet-like visages, and a hunch that would make Quasimodo look like a healthy fellow. They're pretty clumsy at landing or avoiding attacks but can take much more damage than other 'jacks and hit like a Mack truck both at range and up close (just remember that you often have to hit them first).
Among them we find their main workhorse the Juggernaut (bitch!) that you do not want within base to base with anything you own, the Marauder which is so dedicated to smacking people around it has pneumatic ramming prows instead of hands, the Spriggan with its long charging lance and dual grenade/flare launchers, the Decimator with its awesome autocannon/giant circular sawblade combo and the Berserker, which is exactly what you're picturing with: dual axes, EPIC RAGE and tendency to short circuit and explode if you give it too much Focus (that is, any).
The Behemoth, though, exemplifies Khadoran ideals: big, tough, shooty, tough, punchy, big and tough. And like the Thunderhead, it's custom made, making it was too expensive to mass produce, so there's only one available to an army at a time.
Khador has by the coolest looking colossal, the Conquest, carrying a huge fucking gun on its back that actually knocks back stuff in wide area on a critical hit, along with some smaller guns. Compared to other colossal, most players are somewhat indifferent to it, since like the rest of Khador's ranged jacks, it needs support to hit anything, and the Behemoth can do most of the stuff it can for cheaper.
Cryxian light warjacks, called "Bonejacks", are almost all based on the same squat, two-limbed design, earning them the nickname "chickens". The only ones that aren't different models are the burrowing Helldiver, the stealthy assassiny Stalker and Lord Asphyxious' pet scavenger Cankerworm. They are pretty terrible in a straight-up fight and are generally used as vehicles for an Arc Node (a kind of magic repeater that allows warcasters to cast their spells remotely), harassers or suicide attackers.
Cryxian heavy warjacks, called "Helljacks", tend to be fast, hard to hit, relatively fragile, powerful in melee and subpar in ranged combat. Slayer derivatives are similar in design to most other warjacks and offer both simple glass cannons and more support-oriented models, like the harpoon-throwing Reaper, while the Leviathan and its cousins are four-legged weapon platforms with amphibious capabilities. They also have a large range of scary character heavy warjacks, like the Deathjack and the Nightmare, which have their own reasons to be scary as fuck either by generating two free focus or reaping the souls of the dead to buy more attacks.
The Deathjack actually has an interesting backstory, Cryx didn't build it, in fact, nobody knows who built it, they just heard about the thing rampaging around and captured it, and so far haven't been able to copy is soul-fueled engine or dark magic items. Aside from lower armor, the main weakness Cryx's jacks have is that while they are accurate in melee, they're usually as inaccurate as Khador when it comes to shooting.
As befitting for a faction of undead, their style favors the skeletal and menacing, with jet-black paint jobs and lots of spikes. Many cryxian jacks are powered by a necromantic engine that runs on souls and at least three carry particularly nasty chemical weapons.
Cryx's colossal, the Kraken, the least conventional looking of the giant machines before Cyriss' came out, in that has a pair of giant metal tentacles with claws at the end, as well as guns, and a bigger gun on its head. Like other expensive Cryx models, it does something when it kills the living in melee, in its case, it stuffs the corpses of its victims into its furnace that add tokens that can boost its strength, or it can turn them into focus. It has mixed thoughts from Cryx, on one hand, it's one their few good sources of ranged damaged aside from using using warcasters' spells, on the other hand, putting that many points into a single unit tends to going against Cryx's usual thinking that involves having lots of cheap units.
The Nightmare is one of the most grimdark things in Warmachine.
Imperium Protectorate of Menoth faced a difficult problem with psykers Warjacks. Technically they are against the Emperor Menoth's Law since they are powered by vile warpcraft magic, but without them they would be completely buttfucked. So they just rationalize it as a necessary evil, force penitent mages to produce the cortexes and use them anyway. You will immediately recognize a Menite army by the particular blend of mechanical parts and gothic style that wouldn't look out of place in the 41st millennium (also, most of the battlefield will be on fire). Taken alone they are disappointing, mainly due to the use of older models and converted laborjacks, but just add the combo and synergies the Protectorate is famous for and you've really got something. Their light 'jacks tend to be relatively heavily armoured for lights and are either skirmishers (Dervish), spell repeaters (Revenger) or mobile platform for flamethrowers (Repenter) or crude rocket batteries (Redeemer), while heavies are more suited for direct battle. Of particular interest are the Castigator, who just walks up to the enemy and SHORYUKENs him in the face with flamethrower hands and the Sanctifier, a Chaplain Dreadnought who exorcises ghosts and evil spirits on the battlefield (by bludgeoning the HOLY shit out of them with a giant crucifix) There's also the cheap but sturdy Crusader, which is can easily smash much more expensive models to scrap, especially when given some Focus and buffs.
The Avatar of Menoth is, ironically, a Warjack designed and built (through visions to the workers) by Menoth himself. Unlike other Warjacks, it doesn't have a Cortex and can't be controlled by Warcasters. In fact, it operates entirely autonomously and not even the Harbinger of Menoth can control it.
Crunch-wise, it can't be part of a Warcaster's battlegroup and generates it's own focus. It has some interesting abilities and is armed with a fuckhueg sword and shield. It also looks pretty cool and causes Terror. The only real drawback is that it costs nearly as much as two other Warjacks.
The Protectorate's Colossal is the Judicator, which is essentially a giant, walking, mausoleum armed with rocket launchers and flamethrowers. It looks cool, but depending on your taste in asthetics, not as cool as the Khador Conquest.
On the table it's not the worst of the Colossals, but not the best either. It's rocket launchers cause hilarious amounts of explosions, all of which can set fire to whatever they hit when combined with Epic Fiora's Warjack bond. The flamethrowers can however end up being a bit useless in a lot of situations (If you're too far away to Charge, the rockets will be in range. If you're in melee, it's usually better to just bash things with the fists)
Furthermore, Protectorate Character warjacks have the best names in the game, shit like Blessing of Vengeance, Blood of Martyrs and Scourge of Heresy. Suck on that, "Thorn" and "Beast 09".
The Elves of Ios call their warjacks Myrmidons. They are easily the most alien and futuristic in design (to a point were the art for Myrmidons is made of 3D models rather than illustrations). Unlike other 'jacks, Myrmidons are more or less solar powered and come with a powerful forcefield generator that, in addition to protection, allows it to do a number of tricks, ranging from firing laser beams to running far faster than any other jack. The obvious downside is that once the generator has been damaged they're pretty much boned. They tend to dual wield melee weapons with a ray gun of some kind mounted on the shoulder, and have mythological names like Aspis, Hypnos, Sphinx, or Daemon.
Technically each House has a different style of Myrmidons.
House Shyeel uses the force fields while house Vyre lacks them (as well as lights) but are much tougher and dual wield melee weapons with a mounted gun which do a wacky magic trick. (Shyeel Myrmidon's range weapons are formed by its force field).
Mercenary warjacks are almost all old, decommissioned models or armed laborjacks, and it shows. They have low stats like the Protectorate's, but without the Choir of Menoth and their many combos to compensate, and as such they are extremely dependent on focus and their warcaster's buffs to work. They are usually very cheap and do their job decently, without too much flash or show. The Four Star syndicates and the Covenant share most of the same models, chief among them the Renegade, Magnus' pet invention, with a a massive one-shot rocket and a giant chainsaw and the Mule, a jury-rigged mess held together by spit and prayers that nonetheless sports a very powerful steam cannon. The pirates field three converted "sailorjacks": the Buccaneer is a light skirmisher and gladiator wannabee with a net and a gaff; the Freebooter is a weaponized powerlifter that excels at dragging other warjacks to the ground and thrashing them while they're down; and the Mariner couldn't scream "I work on a ship" any louder with its diving-suit-like appearance, anchor in one hand and ship cannon in the other. Rhulic warjacks are slow and sturdy, and include two different moving turrets (one with a big full-auto shotgun), a battering ram with legs and a payload of grenades (so he can explode you while he rams you) and a weaponized excavator with enough strength to punch clean through most other 'jacks with its drill-fist.
Unlike the other factions, there are two mercenary Colossals - The Galleon and the Ghordson Earthbreaker. The Galleon might be the single largest model in the entire game. Its main weapon is a really big industrial claw, while it uses a really, really big harpoon to get enemies close enough to hit with the claw - If it doesn't kill them outright. It also has a lot of deck-guns, which are all AoEs. It is generally known as one of the better colossals. The other is the dwarven Earthbreaker. This machine is actually also made for labor, but fights just as well on the battlefield. Known for its really big grappling claws and its ability to shoot torpedo drills through the palms of them, it moves around the battlefield and forces the enemy to fight at your mercy. Again, though - You have to use one of the dorf warcasters to even field it, and it is very expensive in both money and points, so you don't see it very often.
Cyriss warjacks are called "Vectors", and differ from other warjacks in that they have an "interface node" rather than a cortex due to faction dogma that thou shalt not make a machine in the likeness of a human mind. According to the fluff, this makes vectors easier to control (since the warcaster doesn't have to waste energy getting a separate consciousness to do what they want) but means that without such direct control the machines are little more than decorative statuary. Like Retribution Myrmidons, Vectors do not use steam power, instead relying on internal batteries that are charged by energy fields generated by Convergence temples. Vector designs appear to draw heavily from art deco, and have pretentious techy names like Assimilator, Invertor, and Modulator, because MAGIC SCIENCE!