The Craftworlds are huge planet-sized spacecraft which are home to the Eldar (or Aeldari). Each Craftworld has its own distinct culture, and these Craftworlds house what's left of the Eldar race since an event known as the Fall.
The Eldar are an old race, once masters of the galaxy, long before humanity were even discovering fire. They became decadent in their complacency, and their excess led to the birth of the Chaos god Slaanesh. This event tore a hole in reality, now known as the Eye of Terror, spewing the immaterium of the Warp into realspace.
This divided the Eldar race. There were those who wished to continue their depravity, and they became known as the Dark Eldar, or Drukhari. On the other side were those who wished to repent, and avoid having their souls consumed by the god they had accidentally created.
Thousands of years since the height of their power, the Eldar are a dwindling race, with exceptional craftsmanship of weaponry and armour, and very potent psychic abilities.
Within Warhammer 40k, the Eldar are a relatively fragile but fast army, having access to many specialities of infantry thanks to the different paths of warfare the Aspect Warriors represent. From the heavy ranged firepower of the Dark Reapers, to the horrifyingly quick melee specialist of the Howling Banshees, there's something for everyone.