The quarians are a nomadic species of humanoid aliens in Mass Effect who are known for their skills with technology and synthetic intelligence.
Quarians are bipedal humanoids with three fingers (including an opposable thumb) on each hand and two toes on each foot (which are digigrade). The conditions of the quarian homeworld and the artificially sterile environment of the fleet have left quarian immune systems extremely weak. They must wear environmentally sealed suits at all times, for any foreign microbe can cause intense illness and possible death in a quarian. Because of their constant need to wear sealed suits, their true appearance is unclear to the rest of the galaxy.
In the 19th century (by Earth reckoning), the quarians developed artificial humanoid platforms with virtual intelligence, calling them the geth, which means "servant of the people" in the quarian language. The geth were originally intended to be a non-sapient work force, but they were designed to share data and processing power for greater efficiency. As the geth network grew larger, the geth slowly achieved sapience. Fearing revolt by the geth, the quarians preemptively tried to shut down all geth. This attempt failed and caused the geth rebellion that the quarians were trying to avert. After a brutal war, the quarians were driven from their homeworld, Rannoch, and their major colonies.
The quarians now wander through the galaxy in the massive Migrant Fleet, either seeking a suitable new homeworld or a method to defeat the geth once and for all.