The Animus is a high-tech machine that allows the subject to isolate and decode his or her ancestor's memories and view them in the ancestor's point of view. The Animus requires a live human subject to operate, not only as the source of the DNA (and the memories), but as an necessary agent to interperate the memory. The advanced software then uses the avalible information to create a full stimulation, converting the ancestral memory into a full virtual repersentation. As the subject relives the memory, the Animus captures and records a projection of their experience that is accessible to others as audio-visual media. 
At the same time, the Assassins used the Animus to relive the lives of Desmond Miles' ancestors, training him to become a Master Assassin and to find the Pieces of Eden.