The Geth are a race of networked artificial intelligences that reside beyond the Perseus Veil. The Geth were created about 300 years ago by the Quarians as laborers and machines for war. When the Geth began to question their masters, the Quarians realized they had achieved self-awareness, and thus attempted to exterminate them. A war soon resulted, and the Geth overcame their masters, reducing the Quarians to a race of nomads. The example set by the Geth has led to a legally enforced repression of artificial intelligences in galactic society.
After defeating the Quarians, the Geth isolated themselves beyond the Perseus Veil. They now worship the Reapers, and believe that Saren is the prophet of the Reaper's return to the galaxy. Like other natural races, the Geth can learn and grow intellectually, although in a far more slowly manner.
The Geth were created by the Quarians for cheap labour. They were created as a Virtual Intelligence, however the Quarians continued to make small modifications here and there to enable them to do more complex tasks. The Geth are all connected by a neural network, so when the Quarians built more Geth, their intelligence expanded. One day a Geth asked his master what his purpose in life was.
 Rebellion against the Quarian
The Quarians realised they had created a Artificial Intelligence, which meant that they were now using the Geth as slaves. They knew the backlash this could cause and acted quickly to try and get all Geth shut down. However, the Geth realised what was going on and fought back. The war took billions of Quarian lives and drove the remaining few millions from their home planets. The Citadel races sent their fleets to the edge of the Perseus Veil, expecting a full scale war against the Geth. However, the Geth never left the Veil, they stayed on the Quarian home planets while the Quarians were forced to live in a fleet of starships known as the Migrant Fleet.