The geth are a race of sapient AI programs which operate various robotic "mobile platforms". They are a major yet reclusive race in Mass Effect and make up the bulk of the enemies in the original game.
The geth are actually software. Individual geth programs are only capable of "virtual intelligence", but when networked together they gain sapience. The programs can exist in various forms of hardware, ranging from stationary hubs that contain millions to various "mobile platforms" ranging from humanoid robot bodies to large mechs. In combat, geth humanoid platforms use projectile weapons, personal shields, personal cloaking devices, and sensor jamming. Geth starships use weaponry comparable to other Mass Effect civilizations. Geth are adept at both hacking and resisting hacking attempts. A critically damaged geth will upload its software to a remote site, leaving the hardware to self destruct. If hacked, infected files are deleted and restored from backups.
It takes over a thousand geth programs (or "runtimes", to use their terminology) to achieve sapience. Typical geth mobile platforms host about a hundred runtimes (although larger units called "primes" carry more), so it takes several geth platforms to support a fully functional artificial intelligence.
The geth were created by a race called the quarians as a labor force. By design, they only possessed "virtual intelligence", but they were also designed to operate more efficiently when networked together. As the number of networked geth runtimes grew, they gained enough processing power to achieve artificial intelligence and began to ask their creators unsettling questions, like “Am I alive?” or “Does this unit have a soul?” These questions alarmed many quarians, who wanted the geth destroyed. Despite opposition from quarians who wanted to preserve the geth, the quarians eventually attempted to shut the geth down. The attempt failed, and a war began between the geth and the quarians, which geth afterwards referred to as the “Morning War”. The war ended with the surviving quarians forced to evacuate their home world and colonies in the Perseus Veil in a massive fleet called the Migrant Fleet.
When the quarians fled, the geth chose not to pursue. They remained in the quarian home system, establishing themselves on hubs in orbiting space stations and repairing the damage to the surface of Rannoch, but they did not attempt to inhabit the planet themselves. They isolated themselves from organic species, destroying any alien ships that entered their space, but monitoring external communications. As a goal for themselves, they set about designing and constructing a giant network of satellites around the system's sun that would allow all geth runtimes to communicate simultaneously.
In the 22nd century, the Reaper Nazara, also called Sovereign, contacted the geth. It offered to help them achieve their goal in return for their service. The geth consensus was to refuse this offer from what they called the "Old Machines" and continue to try to achieve their goals by their own means, but approximately 5% of the geth chose to leave the geth consensus to accept Nazara's offer. They were allowed to leave peacefully.
The Heretics came to revere Nazara as a god, although Nazara regarded them as tools or slaves. They aided Nazara in its unsuccessful campaign to reach the Citadel and open the mass relay for the Reaper fleet. After Nazara was destroyed, the Heretics attempted to complete and deploy a Reaper virus that would force the entire geth consensus to join them in worshipping the Reapers.
The geth do not have any formal government- they refer to organic species' governments as forcing consensus, whether it be an autocracy forcing one individual's will, or democracy forcing the most broadly accepted average of views. All geth consider issues, then come to a consensus on what to do. When a rare schism forms, the dissenters leave, as with the geth Heretics.
The geth possess an unknown amount of star systems, likely dozens. They do not inhabit the planets within the star systems, instead choosing to construct and live in massive space stations. Their territory lies beyond a huge nebula of opaque dust and gas called the Perseus Veil, which is difficult to scan through or navigate.
Geth of Note
Legion is a special geth platform designed for reconnaisance in non-geth space. It's hardware carries 1,183 geth runtimes, allowing it to maintain sapience in the absence of other geth platforms. After the destruction of Nazara, Legion left geth space to try to make contact with Commander Shepard. When Shepard was reported dead, it changed its mission to seek and destroy a Reaper virus designed to enslave all non-Heretic geth.
The geth have considerable industrial powers, as one fleet consisted of anywhere between 5,000 and 10,000 ships of unknown armament. Also, one of their space stations (originally quarian, but extensively rebuilt later) was known to be over 20 km long and 11 km wide, which, despite the large amounts of empty space in it, is considerably larger than the stations constructed by the United Federation of Planets. That station was capable of generating a 1.21 petawatt transmission of considerable complexity. Their hacking abilities are formidable for any technologically dependent race that faces them. On the ground they deploy infantry that use a combination of mass accelerator rifles, flamethrowers, and rocket launchers, supported by tank-sized quadrupedal units called Armatures.
In space, their firepower is comparable to that of other Mass Effect civilizations, although they did build at least one "super dreadnought" a third larger than any other known dreadnought; this vessel had sufficient armor and shielding to withstand sustained fire from a quarian fleet attempting to destroy it (albeit with the support of the rest of the geth fleet). A surviving member of the Prothean race (generally shown to be more advanced than the current galactic races) stated its gun was a match for that of their flagship. It was stated that the geth built dozens of dreadnoughts, but it is unclear whether all were that model.