The Eldar are a race of Elf or fey-like humanoids, and they are the most ancient and technologically advanced of all the intelligent races of the Milky Way Galaxy,
with the exception of the Old Ones, the Necrons and their C'tan masters.
Their armies usually have the advantages of great mobility, potent psychic abilities and technology that is more advanced than that deployed by the human Imperium.
In the sense that Warhammer 40,000 races are generally derived from the earlier Warhammer Fantasy universe, the Eldar are derived from the High Elves of the Warhammer World,
except for the Eldar faction known as the Exodites who are the equivalent of the Wood Elves.