The Geth are a race of really awesome robots. They have flashlights for eyes and really cool voices. Geth were created by a gang of Immigrants who lived on a Giant Garden and couldn't be bothered to water their plants. So they got Geth to do it for them.Fortunately, because Geth are so awesome, they developed sentience, turning from VIs into AIs. They were very nice to their creators and offered to help of their own free will, rather than being forced to. The immigrants, however, got scared that the Geth were sentient and tryed to wipe them out. However, also because the Geth are so awesome, they kicked the immigrant's arses and booted them off the planet in the Morning War. Becuase they are so nice, they let the immigrants evacuate and Geth commanders issued orders not to fire on civilians or their transports.
Geth were once a networked intelligence. But during the Reaper War, one particularly awesome Geth called Legion sacrificed himself, uploading his conciousness to all Geth. This allowed them to achieve individual sentience without needing to link their neural networks with other Geth.
Like the Covenant, Geth were divided into Heretics and Loyalists. Unlike The Covenant, the Heretics are the "baddies" and the Loyalists are the "goodies". Heretics worshipped the Giant Squids and Loyalists didn't. Also unlike The Covenant, there are much more Loyalists than Heretics. This fracture in Geth society basically ended after the Reaper War, with Heretic Geth reintegrating themselves into sociey. They now all have respect for Reapers, without going to the extremes of worshipping them.
The Geth serve mainly as accountants and builders in League society, but there are also a few very skilled Geth chefs hanging around. They get on well with Elites for some reason, and they also respect Rarth Devan for his great strides forward in AI Rights campaigning.