Knightly Orders of the Empire
The Knightly Orders of the Empire are... pretty much what it says on the tin, Orders of Knights. Formed almost exclusively from nobles or higher born members of the Empire (because why the fuck would you want to hang out with dirty commoners), the Knightly Orders are separate organisations to the standing armies of the Empire and are used by nobles as a means to find military glory, or further their carriers in politics or as generals in the Empire. Since the Knightly orders are outside the general jurisdiction of the standing armies, knights can go pretty much anywhere in (and outside of) the Empire and can generally fight who and where they choose, though the Elector Counts and Emperor can order knights to battlefields where they are needed. The Knights are typically self supplied and as a result are far better equipped compared to your standard state trooper, since some rich noble won't want to risk his life mucking about with gambeson and a crude sword, so each knight is equipped with the best armour and best weapons he can afford, for both him and his horse. Full plate, Shields, Barding, Lances, the works. Empire Knights are so heavily armed the only other things that come close are Chaos Knights.
An Order has their own Chapterhouses which they maintain and which house the serving members of the order, with orders having over a dozen Chapterhouses throughout multiple provinces, to just a single house/fortress out in the boonies, and Orders are not actually restricted to the Empire and can have chapter houses or even be based in other countries. Each order has their own beliefs, traditions and duties with some orders having certain tasks that they have bestowed upon themselves or were founded to do, such as guarding gravesites or patrolling part of the Empire boarders and other orders which are dedicated to the worship of different gods, with Orders that dedicate themselves to a deity being known as Templar Orders. And while typically if asked a Knightly Order will send knights, these traditions can make dealing with certain orders "difficult" as an order can have dozens of reasons and excuses for not helping a general and doing something else. Though if one order won't help you, a different one probably will.
The Grand Order of the Reiksguard Knights is sworn to protect the Emperor and the Imperial bloodline. Based in Reikland (duh), their main Chapterhouse is found in Altdorf, with smaller houses found throughout the rest of the province allowing them to react quickly to threats to the province. They take active participation in Reiklands armies and members will act as advisors and bodyguards for Generals as well having marshals leading armies at times as well. They have no real specialisation apart from being very elite and prestigious, even among the other Knightly Orders. Their Grand Master is Kurt Helborg.
Knights of Morr
Also called the Black Guard, this order dedicated to Morr specializes in killing undead and guarding the graveyards of the Empire and other countries. Their main Chapterhouse is in Luccini, a city in Tilea, though they have smaller chapterhouses in the Empire, and the Knights are noted as having a very grim attitude, who are sworn to silence when on duty, though they can speak during combat. Their armour is made out of pure obsidian (don't ask me how) and is padded to mean they make very little noise when walking around, adding to their "silent as the grave" thing they have going on. They also are permitted to use ranged weapons like crossbows and handguns as they commonly fight Necromancers and Vampires, where it's more effective to shoot them to weaken or outright kill them, before charging into combat.
Knights of the Blazing Sun
Founded just before the the Great Crusade against Araby, a group of Knights were stationed in Estalia when the armies of Araby attacked the city of Magritta. These 60 knights became trapped in a temple of Myrmidia and were pretty much done for, until an act of god(dess) occurred. An Earthquake hit the city and dislodged a statue of Myrmidia which crushed the Araby soldiers and their general as they were trying to get into the temple and allowed the Knights to escape and turn the tide of the battle as well as evacuating over 700 civilians. Believing that the Earthquake had been sent by Myrmidia the Knights dedicated themselves to the goddess, rather than blame the thing on dumb luck and shitty Estalian stone masonry, and took part in the Araby Crusades as revenge for the invasion of Estalia. They wear Golden Armour (or Black/Silver with gold highlights depending on your Vidya of choice) with the sun being a reoccurring symbol on their armour, and they have two main Chapterhouses with one in Talabecland and the other in Averland. Despite this, the knights will actually spend most of their time away from their Chapterhouses, typically travelling around the Empire and other parts of the world in groups and offering their services and aiding towns under attack which makes them a very widespread order and one of the fastest to react to threats to the Empire, such as when Asavar Kul started his murder party in Kislev during the Great War against Chaos, they were one of the first Knightly Orders to get a sizable force across the boarder into Kislev to fight the Chaos Hoards.
Knights of the White Wolf
Basically the fantasy equivalent of the Space Wolves, but cooler and with less of a Furry fetish. These knights come from the cult of Ulric and wear wolf pelts, which they need to go and hunt down themselves to join the Order. Magnificent beards are also required for entry. They are actually the oldest knightly order in the entire world, having been founded 2,500 years ago when Sigmar ruled as the first Emperor and to no surprise they are based in Middenheim with most of Ulrics other worshippers. They differentiate themselves from other Knightly Orders by not wearing helmets or using shields, but instead using two-handed Warhammers that they use from horseback to dome in the heads of the Beastmen that they are running down. This makes them more vulnerable to attacks than other knights, but the pay off is that they are far more brutal after the charge and in the ensuing melee as they swing their hammers round sending bones, teeth and grey matter flying as they replace the childhood memories of their opponents with blunt steel. Due to their worship of Ulric the knights guard temples of Ulric around the Empire, not just in Middenland and they are loyal to the Ar-Ulric (the Wolfman pope) acting as both his bodyguard and agents who go around spreading the good word of Ulric. The word is usually "THWACK!"
Established after the Great Crusade against Araby and named after the exotic panther pelts that soldiers brought back from Araby, these knights are dedicated to upholding the racial purity of the empire by killing mutants and as such have a colossal hate boner for Chaos. They are constantly sending groups of knights into the Drakwald and other forests to flush out the Beastmen and Greenskins that reside in them and scour them from the Empire. They maintain a fort near Carroburg, as well as over a dozen smaller chapterhouses throughout the Empire and two large chapterhouses in Middenheim and Talabheim. They are very recognisable as they like to keep up a rather splendid appearance with large ornaments on their helmets and their exotic animals skins they wear as cloaks, just to help them stand out against other knights and show how much of a bunch of poncy assholes they are. That said they still can fight very well and the Knights Panther have been instrumental in halting many threats to the Empire, including the recent Orc rompfest that was Azhag, when the Grand Marshall of the order slayed the warboss, though he was distracted at the time.
Other Knightly Orders
Foppish rich dilettantes. They only take part in campaigns that will give them the most glory while requiring the least actual work. They specialize in a unique technique of dual-wielding swords on horseback.
Knights of Sigmar's Blood
Knights in training must spend a year in study at the church of Sigmar
Knights of Taal's Fury
They all ride demigryphs instead of horses. Dedicated the god Taal (who would've guessed?). New initiates must go into the forest and capture a wild demigryph to be their mount.
Knights of the Black Bear
Stationed in Averland. Each of this order's tournaments opens with the Grand Master wrestling a bear. A practice that has led to the death of many Grand Masters. It's rumored that they are named after a tavern in Averland, but if one of the knights hears you say this you will get bisected.
Knights of the Black Lynx
Stationed in Black Fire Pass. Unlike other knights, they rarely fight on horseback, prefering instead to silently sneak up on enemies on foot. The order was created by Marius Leitdorf to honor the 9 men out of 400 who survived a battle against 3,000 orcs.
Knights of the Broken Sword
Started as a group of freelancers gathered by Dott Barthos to hunt down a band of Beastmen led by Sizlak Grimhoof. The cleft-in-twain skull of Grimhoof and the broken sword of Barthos are kept in the chapterhouse in memory of their victory over the beastmen.
Knights of the Everlasting Light
Dedicated to Verena. The entire order has been cursed so that its members will only die ignoble deaths.
Knights of the Fiery Heart
Personal bodyguard of the Grand Theogonist (sigmarite pope). Every few years a member quests to Karak Eight-Peaks to find the legendary sword Karaghul. What they don't know is that Karaghul has already been nicked by Gotrek & Felix.
Knights of the Gold Lion
Founded during the Great Crusade when a knight tried to cheer up his companions after a crushing defeat by wandering into the savannah, killing a lion, and bringing back its head as a sign of Sigmar's favor.
Knights of the Sacred Scythe
Vampire hunters based in Eastern Stirland.
Knights of the Vengeful Sun
They also only ride demigryphs. Founded during the Vampire Wars.
Knights of the Verdant Field
An order created by Fantasy Flight Games for the Warhammer RPG. They are dedicated to Myrmidia and tasked with protecting the city of Talabheim in Talabecland. They don't wear platemail, instead dressing like a steretypical Ranger or Druid.