Geth

&nbsp The Geth are a race of networked artificial intelligences that reside beyond the Perseus Veil.

While the Geth can learn and grow intellectually, they progress far more slowly than organic beings typically do. Even so, all Geth should be approached with extreme caution, as each and every subtype is equally and universally violent.

Physiology
Likely due to a holdover from their origins, Geth slightly resemble their Quarian creators, most prominently in the legs, although perhaps even moreso as Quarians keep their faces masked at all times due to their weak immune systems. They are built of two materials: a large plastic or steel outer casing and a kind of synthetic musculature tissue that is far superior to organic muscle (this is proven true as it is what gives the Geth Hoppers their incredible agility). Many sapient citizens describe the Geth as having "flashlight heads" due to their large glowing ocular lens. If injured, a Geth may "bleed" white conductive fluid, but they do not have any internal organs or even a nervous system, and therefore they do not feel hunger or pain in any way. This ability actually allows them to literally drop their soldiers in from above as if out of nowhere. They can also be packed in crates to be shipped into position.

A large part of the Geth's success is due to their neural network. This networking means that the closer each Geth physically get to one another, the more intelligent each one actually becomes. Effectively, they 'share' their processing power. At the same time, however, Geth cannot share sensory data - they are not a hive mind like the Rachni - but in large groups they have more to think with, and large, close-knit Geth groups can be a force to be reckoned with. The individual Geth has only a basic intelligence on par with animal instincts, but in groups they are able to reason, analyze situations, and use tactics as well as any organic sapient. Apart from their obedience to their 'gods', the Reapers, nothing is known about the geth command hierarchy, or even if they have one at all.

When geth shut down, they fry their memory cores as a defensive measure, which is quite likely why Geth haven't been successfully captured for study. However, a Quarian named Tali'Zorah vas Normandy was able to retrieve some of the data cache from a deactivated Geth, through the use of her technical expertise.

There are several types of Geth the main types being Trooper, Shock Trooper, Armiture, Stalker,and Destroyer.There are also expanded versions of those Geth such as the Collosus, the Juggernaut, and the Geth Prime. New forms of geth in Mass effect 2 now include the standard while a new subsect is now known, the Hunters, who are equipped with a cloaking field, near constant kinetic barriers and a shotgun, making them a either a dangerous foe or a annoyance depending on what difficulty level you play at.

While the statmeent above about the network is true, it is apperently possible for multiple geth personas (or runtimes as Legion put it) to be uploaded into a single geth hardware platform (a geth body) for extended solo missions as seen with the potential squadmate in Mass Effect 2, Legion. It should be noted that, though synthetic in origin, geth are nearly impoosible to hack, as with most hackings, you are adding new programs, which the geth almost immediately delete to restore themselves. This system is not failproof, as if they are hacked successfully, they make potent allies or basic cannonfodder.

Geth minds, because of their synthic origin, do not proccess as other races, with their decisions being made using mathematical computations, as seen in Mass Effect 2 when discussing the Heretic faction of the geth with Legion. It seems that, through subtle changes in basic programs, decisions that the geth make can be altered and changed, as seen when the Heretics deicded to follow Nazara (Sovreign)

History
the Geth were an experiment in AI research by the Quarians to act as a labour force for their civilisation.

Unfortunately, when the Geth became sapient and began to question their masters, the Quarians  became scared of what might happen and attempted to exterminate them. This set the Geth to defend themselves, starting a bloody war that ended in the Quarians being pushed back from their own homeworld, setting them on a path of nomadic life from then on. Because of the example of how synthetic sapience wars could begin and play out, they unintentionally caused the Council races to legally enforce the systematic repression of artificial intelligence in galactic society.