The Geth are a synthetic race featured in the Mass Effect trilogy. They were originally created by the quarians as a manual labour force, but eventually developed sentience and rebelled against their creators, exiling them from their homeworld Rannoch and forcing them to live aboard the Migrant Fleet. They appear as enemies in all 3 Mass Effect games.
The Geth (Khelish for Servant of the People) were originally created by the quarians as a labour force designed to operate more efficiently when networked together. They initially started out as simple VI's, with no real sentience, but over time their creators upgraded their progamming, resulting in them becoming a sentient collective of AI's. When they began to question their creators (more specifically asking if they had souls) the quarians panicked and attempted to exterminate them. The geth retaliated, and eventually won the resulting war, killing millions of quarians andforcing the survivors off of Rannoch aboard the Migrant Fleet. Once the quarians fled the system, the geth did not pursue, and instead remained on Rannoch, and reamined quiet for the next two centuries.
In the first Mass Effect, the geth attack the human colony of Eden Prime, led by rogue Spectre Saren Arterius. It later transpires that the geth are helping Saren brign about the return of the Reapers, which they worship as gods. The geth follow Sarens orders without question, believing him to be the Reaper's herald due to him piloting the ship Sovereign (which is a Reaper itself)
In Mass Effect 2, a docile geth dubbed Legion is encountered. It reveals that the geth that followed Saren were a small percentage of the geth collective, dubbed 'heretics' by the main geth who refused Sovereign's offer to join it. In Legion's own words, the heretics want the Reapers to give them the future, while the main geth wish to build their own future.