The Warlord is a Dungeons & Dragons character class introduced to the game in Dungeons & Dragons 4th edition. Appearing in the first Player's Handbook, it was a flagship for the new design style of character classes in 4e, which in many ways makes it amongst the most skubtastic class of an already skubby edition. Although the exact name first appeared in 4e (and the mechanics are so full of 4e-isms that its inseparable) the class was based on the Marshal class that appeared in the 3.5 Miniatures Handbook.
In a nutshell
The Warlord represents various "commander" archetypes, from a tactical genius to a charismatic gangboss to a grizzled veteran to a former army official. In the terminology of 4e, the Warlord is a Leader class of the Martial power source. This means it has the tactical role of "Leader", a secondary combatant focused primarily on augmenting the abilities of other party members, and it belongs to the "Martial" power source, which means its skills derive from basic training and combat savvy, rather than anything supernatural.
How's that work?
All Leaders are designed with the intent of supporting the party as a whole, but each Leader has its own way of doing so. Being a Martial class, the Warlord is a subpar healer, because it relies heavily on the tradition of hit points "abstractly" representing the health of a character. A Warlord's healing is not so much "magic somebody's arm back on" as "shout at somebody to stop being such a wuss and get back in there, Drill Sergeant style". Healing is left to its traditional master, the Cleric.
Where the Warlord excels as a Leader is in its tactical boosting. The Warlord specializes in handing out initiative bonuses, extra moves and extra attacks to the rest of the party like candy. A popular theoretical build, affectionately nicknamed "the Lazylord", actually has no direct attacking powers whatsoever; all of its powers revolve around maneuvering the rest of the party to counter threats. Hence the saying "A barbarian hits you with his axe; a warlord hits you with his barbarian.'"
Although this is directly handled by the various Warlord exploits (special attacks), it's also influenced by the class features. Keymost to these is the Commanding Presence feature, which gives a different bonus to allies who spend an action point within its area of effect: a Bravura Presence gives them increased attack options with penalties for failure, an Insightful Presence grants a defense boost, an Inspiring Presence grants free healing, a Resourceful Presence grants either a damage buff or free healing depending on the situation, a Skirmishing Presence lets them take a free move, and a Tactical Presence grants increased attack. The other feature, Combat Leader, gives your party a much-needed bonus to Initiative, which can help a striker or controller throw out their best stuff first, while its alternatives give either bonuses to scouting (Canny Leader) or give proficiency with shields and the ability to move one ally quickly before the fight even begins (Battlefront Leader).
The Warlord isn't good for players who want to be "The Hero", but then, the Leader role as a whole doesn't fit that goal very well. No, the Warlord is for players who want to be useful to the team as the whole. The player who has the most fun with the Warlord is generally somebody who likes the idea of not only getting to play a flavorful character, but also to help the team as a whole succeed.
And make no mistake, the Warlord can be pretty fun. Want to get your Drill Sergeant on? Warlord works wonders for that. Want to be a grizzled dwarf longbeard grumbling about how kids today don't realize how good they got it and generously sharing your wisdom so they don't screw up quite as often? Warlord can do that. Haughty nobleman or knight? Warlord. Napoleon? William Wallace? Both Warlords.
Hey, in playtesting, they had a power that let the rest of your party get a free shooting attack against a designated target called "Feather Me Yon Oaf". That's got to count for something!
Warlords, as mentioned above, are pretty much the most controversial class in 4e, because they were brand new. Their fans absolutely adore them for the flavor they bring and because they were the first ever real attempt at a fully non-magic healer/buffer class. Others mocked them, and often were mocked in turn because their arguments tended to be repetitive ("how does it heal without magic? what if my PC doesn't want to take orders from anybody?") and usually pretty easy to shoot down.
|Dungeons & Dragons 4th Edition Classes|
|Player's Handbook 1:||Cleric - Fighter - Paladin - Ranger - Rogue - Warlock - Warlord - Wizard|
|Player's Handbook 2:||Avenger - Barbarian - Bard - Druid - Invoker - Shaman - Sorcerer - Warden|
|Player's Handbook 3:||Ardent - Battlemind - Monk - Psion - Runepriest - Seeker|
|Heroes of X:||Blackguard - Binder - Cavalier - Elementalist - Hexblade - Hunter|
Mage - Knight - Protector - Scout - Sentinel - Skald - Slayer - Sha'ir - Thief
Vampire - Warpriest - Witch
|Settings Book:||Artificer - Bladesinger - Swordmage|
|Others:||Paragon Path - Epic Destiny|
Though they share a name, the Warlords found in Pathfinder are a completely different class compared to the one above, the most obvious being that this one is directly tied into the Path of War system that 3PP Publisher DreamScarred Press ported over from 3.X. The extent to which this Warlord is a leader is tied to one particular discipline as well as teamwork feats, which can only be shared with allies after level 3. This class also has absolutely no healing power.
The first big mechanic this class has besides the maneuvers are Gambits, which are various risky tricks that the Warlord can attempt (with a very minor luck bonus) to not only recover any expended maneuvers, but also for a certain other benefit associated with the gambit. Failing lets them recover a maneuver as well, but it also penalizes them hard with a -2 on all rolls for a turn. Their next big thing is Tactical Presence, a set of auras that spread benefits to allies within a set range. At level 11, two of these can be stacked together and used simultaneously, while 15 ups this to three.
Then there are the minor features that pepper this class. Stuff like "acting like you occupy a space near you to assist with flanking" or stacking boosts and stances, which is unique and helpful for a class built around assisting allies.
Their few archetypes do add some special thematics as well, from the Bannerman's flag that actually does stuff, the Desperado's Gunslinger-like gunplay, Steelfist Commando's fisticuffs, or the Vanguard Commander's steel-plated gusto.
|The Classes of Pathfinder 1st Edition|
|Core Classes:||Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Wizard
|Alchemist - Antipaladin - Cavalier |
Inquisitor - Oracle - Summoner - Witch
|Arcanist - Bloodrager - Brawler - Hunter - Investigator |
Shaman - Skald - Slayer - Swashbuckler - Warpriest
|Kineticist - Medium - Mesmerist |
Occultist - Psychic - Spiritualist
|Ultimate X:||Gunslinger - Magus - Ninja - Samurai - Shifter - Vigilante|
|The Third Party Classes of Pathfinder|
|Psionics:||Psion - Psychic Warrior - Soulknife - Wilder |
Aegis - Cryptic - Dread - Marksman - Tactican - Vitalist
|Path of War:||Stalker - Warder - Warlord - Harbinger - Mystic - Zealot|
|Akashic Mysteries:||Daevic - Eclipse - Guru - Nexus - Radiant - Rajah - Stormbound - Vizier - Zodiac|
|Spheres of Power:||Armorist - Elementalist - Eliciter - Fey Adept - Hedgewitch - Incanter |
Mageknight - Shifter - Soul Weaver - Symbiat - Thaumaturge - Wraith
|Spheres of Might:||Armiger - Blacksmith - Commander - Conscript |
Savant - Scholar - Sentinel - Striker - Technician
|Champions of the Spheres:||Prodigy - Sage - Troubadour - Dragoon - Mountebank - Necros - Reaper - Warden - Crimson Dancer|
|Pact Magic:||Pactmaker (formerly known as Medium)|
The Warlord has essentially been lost with the changing of editions. The Battlemaster and Banneret subclasses for the Fighter arguably touch upon the same themes, and the Oath of the Crown Paladin and College of Valor Bard can both make decent-ish substitutes, but for most Warlord fans they're just not good enough, and the Warlord remains a surprisingly common request from WoTC despite how hated 4e was during its lifetime. This is probably due to the same reason Dragonborn aren't hated (as much) anymore: They both scratched an itch D&D roleplayers didn't know they had; in this case, "Officer" as distinct from "Warrior".
In addition to all this, as part of a livestream by Mike Mearls where he spitballs ideas for new subclasses included an episode where he made a Warlord archetype for Fighters with Tactics to improve allies and Gambits to soften up enemies. It hasn't been seen since, however.