Craftworld
Craftworlds are massive, self-sufficient Eldar vessels, essentially mobile planets made of Wraithbone flying through space.
History
During the height of the Eldar Empire, the Eldar built the Craftworlds as large merchant ships. Somewhat questionably for traders, they didn't install any sort of Webway or Warp drives on the Craftworlds, instead forcing the Craftworlds to travel at sublight speed. Ultimately, this would be to the Eldar's benefit, for the amount of time spent between Eldar worlds allowed the Eldar on the Craftworlds to see just how far their people had fallen into hedonism. Realizing that something bad would happen soon, the Craftworld Eldar evacuated the Eldar still sane enough to realize how far they'd fallen from their depraved brethren, and fled, taking with them the repositories of Eldar history, science, religion, and pre-Fall culture.
Unfortunately, not all of them made it out of the Segmentum Obscurus in time, and many of the Craftworlds were caught in the creation of the Eye of Terror just as the hedonists were. So grimdark.
Today, the Craftworld Eldar comprise the largest populations of surviving Eldar. They have similar goals in preserving the Eldar, reestablishing their Empire, and fighting Chaos. However, the Craftworlds often disagree over how to achieve these things. This can lead to small conflicts between the different Craftworlds, but for the most part these are rare.
Tabletop
Back in the third edition of Warhammer 40,000, Games Workshop released Codex: Craftworld Eldar, a "mini-dex" expansion booklet for the Eldar codex. It included five army list variants for the craftworlds Alaitoc, Biel-Tan, Iyanden, Saim-Hann, and Ulthwe. Each Craftworld receives a few unique units, and some receive army-wide special rules, a re-assignment of the available squads between the various categories (Troops, Elites, etc.), or even a different force organization chart. In the later Codex: Eye of Terror (released for the 2003 "Eye of Terror" summer campaign), Ulthwe got another mini-list for their "Strike Force".
Unfortunately, the Eldar got hit with the "less-is-more" bat in Fourth Edition, so the only difference between the Craftworlds on the tabletop throughout Fourth and Fifth Editions was that they had different colors and symbols. However, with the new Sixth Edition Codex, Games Workshop has bowed to fan demands seen an opportunity to make more money, and released a codex supplement all about Iyanden (unfortunately, it was written by none other than Your Spiritual Liege, and it shows). With any luck, the other "big five" Craftworlds (and similar supplements for other codices, perhaps) are on the way, ideally with more competent writers.
Notable Craftworlds and specialties
The big five
- Alaitoc -- Follow the Eldar Paths more strictly and therefore have less fun. This leads many to become arrogant outcasts and eventually veteran rangers and pathfinders. They are also archenemies of the Necrons, and fight the zombie-bots on a constant basis.
- Biel-tan -- Wish to reclaim the old Empire of the Eldar before it got screwed over. Almost everyone is arrogantly warring with the lesser races. They want to rebuild the Eldar Empire and believe anyone else who even touches a Maiden World are squatters to be removed. This Craftworld is notable for being even more arrogant than most other Eldar. Even other Eldar have difficultly dealing with them. Biel-Tan is fiercely protective of Exodite and Maiden worlds, and are known to reserve a special hatred for Orks, who are especially hard to exterminate from any planet they've managed to get a foothold on. In the original Dawn of War, Macha was from Biel-Tan (although current /tg/ canon suggests she spends much of her time on Ulthwé, due to Biel-Tan's lesser respect for Farseers).
- Iyanden -- It used to be the largest Craftworld, but then it got OM NOM NOM NOM'd so almost everyone is dead. Fields lots of Wraith-units to make up for the whole "barely anyone left alive" thing. Known for their Spiritseers, necessary for an army like theirs.
- Saim-Hann -- Jetbikes all day erryday. Considered barbarians by some other Craftworlds.
- Ulthwé -- They're a few inches away from the bleeding asshole of reality (which is why it's nicknamed "Ulthwé the Damned") so everyone is extensively trained for war, even the regular Guardians, who are called "Black Guardians". Also, this is where that dick was born. Also home to Caerys, Idranel, and Taldeer from the Dawn of War series.
All the others
- Altansar -- The home of Phoenix Lord Maugan Ra. Once thought lost in the Warp, Maugan Ra managed to find them and guide them out. Not very liked by the other Eldar, because they (rightfully so) think that nothing can remain untouched by the Warp. But don't mention that around Ra if you want to avoid repercussions.
- Idharae -- Teamed up with Malan'tai and Alaitoc to fight Tyranids before Malan'tai got Doomed, then got themselves destroyed by the Invaders Space Marines. The invaders also had the help of the legion of the damned and an unknown number of imperial assets. The invaders also only attempted to attack after the craftworld had just fought off Hive Fleet Naga and was still in a weak position.
- Il-Kaithe -- On an eternal crusade against Chaos, they are unusual in that they take their Bonesingers into battle.
- Iybraesil -- A matriarchal society that likely employs large numbers of Howling Banshees. They have a great interest in the Crone Worlds.
- Lugganath -- BFFs with the Harlequins, they think that the Galaxy has gone down the crapper (no shit, Sherlock) and want to start anew in the Webway after ridding it of the Dark Eldar (which is not likely to happen). To do this, they're trying to find a Craftworld-sized webway portal.
- Kher-ys -- Destroyed by a Slaaneshi daemon incursion. Oh, and a Keeper of Secrets possessed their Avatar during the fighting, you can imagine how much
proverbialbuttrape happened there. - Malan'tai -- Got LOLDRAIN'D by a unique Zoanthrope who feeds off psychic energy. Needless to say, after it devoured the souls on the Craftworld's Infinity Circuit, things went very bad for them. Are now extinct. The Grey Knights under the command of Anval Thawn went and saved the remaining soulstones form a Keeper of Secrets and were uncharacteristically generous enough to give them back to the Eldar.
- Mymeara -- Has an Imperial Armour book named after them detailing their fall. Also home to the Phoenix Lord Irilyth and the Shadow Spectre Aspect Warriors.
- Yme-Loc -- Munchkin motherfuckers who spam grav-tanks and Titans in battle. Unluckily for them, the Adeptus Mechanicus made off with several of their toys.