Planet of origin: Unknown, in the Milky Way galaxy
The Eldar are an ancient race of tall and slender extremely very long-lived human-looking humanoids, with much faster metabolic rates than humans, as well as better reflexes and a superior intellect. They were created by the Old Ones some 65 millions years ago to serve in their War against the C'Tan and their Necrons servants. Following the departure or disparation of their makers at the end of the war, they became a dominant species in the Milky Way galaxy. They possess strong psychic powers and have a fair amount of their population endowed with some level of prescience.
It is so believed that within the upcoming 28 000 years, they will come close to extinction, with survivors existing in a few wondering craftwords, planetoid sized spaceships, hiding among the stars. The reason of their fall will come from within, as consumed by arrogance and with no need for substantial work , the pursuit of their desires will lead them toward a path of instant fulfillment and unbridled violence. The psychic emanations of these activities will give the Eldar vices a life of its own, a consciousness known as Slaanesh, the Devourer of Souls, whose psychic scream of birth will tore the souls from all the Eldar within a thousand light years of it and plunge the Eldar homeworlds into a limbo of partial existence, a region which will become known as the Eye of Terror, a bridge between real space and the Warp, the psychic imprint of all the souls within the Milky Way.
Much of Elda culture is based around their mythological cycles. The most famous of these cycles was the War in Heaven, an epic conflict between the Eldar deities, in two factions lead by Vaul, the god of the forge, and Kaela Mensha Khaine, the god of war, which is based on the war the Old ones led against the C'Tan and their Necrons servants some 65 millions years ago.
Eldar technology is notable for its use of Spirit stones, wraithbone and the Webway.
The Eldar use spirit stones to capture, contain and protect their souls at the moment of death. These stones are then collected and inserted into their Craftworld's "Infinity Circuit", where they may rest along with the spirits of their ancestors. In times of need, the spirit stones of the Craftworld's strongest warriors may be taken from the Infinity Circuit and placed inside Wraithbone automatons to once again fight for the Craftworld.
Wraithbone is the main construction material of the Eldar, and the staple of their psycho-technic engineering. It is pyschic energy solidifed into mattter through psychic power. It is a psychic conductor and so not only provides the structure for the things built of it, but also power distribution and communications. Wraithbone is a highly resilient material, and capable of limited self-repair. It, and the other building materials of the Eldar, will grow and react more like tissue and plants than the building materials of other races. Every Eldar Craftworld contains an Infinity Circuit, which is the Wraithbone skeleton of the Craftworld . Within this matrix the souls of all the Craftworld's dead reside in a form of group consciousness, providing both a well of psychic power for the ship and a massive ancestral mind to advise and guide the living. The Infinity Circuit is the closest thing that the Eldar have to an afterlife; if their souls are not caught and integrated into it, they will be lost into the Warp and subject to its perils. For this reason the Eldar will defend their Craftworlds with a fury and tenacity almost unrivaled; they risk losing not only their home but their souls and that of their ancestors.
The Eldar travel through the galaxy by use of the Webway, a system of ancient "tunnels" through the Warp but completely isolated from its inherent dangers. It is a vast and tangled network of doorways between fixed points in real space, by which the Eldar can travel far more rapidly than most races.
Source: Games Workshop's Warhammer 40,000 universe.