The Animus is the memory machine used in Assassin's Creed. It allows a person to access genetic memories inherited from ancestors and experience them as interactive simulations.
The technology frames much of the series' modern-day storyline. Desmond Miles uses an Animus to relive the lives of Altaïr Ibn-La'Ahad, Ezio Auditore, and other ancestors. Abstergo originally develops and controls the device, while later Assassin operations use modified versions for their own investigations.
The Animus does not literally send a user into the past. It reconstructs ancestral memories from genetic information, and the series presents the resulting experiences through a game-like interface. Helix is a later entertainment-oriented platform related to Abstergo's technology, whereas Eagle Vision is an Assassin ability rather than a machine.