Geth is actually a race that created by Quarian people. All Mass Effect fans could probably know Geth as it appeared since the first Mass Effect game released. Originally, Quarian creates Geth to become laborers as well as tools of war. However, Geth eventually become sentient and began questioning the Quarians. Avoiding the worst, Quarians tried to exterminate all the Geth, but in then the war was won by Geth. That is the reason why quarians finally become a race of nomads. Along with the success of Geth take over the control from its creator send a great warning for the rest of the galaxy the danger of the artificial intelligent. That’s what the main story and background of Geth in Mass Effect.
Continue on, the story told that Geth have evolved in various sub forms, with each advantage for each sub forms. However, since it is always appear in previous installment of Mass Effect series, including Mass Effect and Mass Effect 2, it is no wonder that Geth also appear in the latest series, Mass Effect 3. In Mass Effect and Mass Effect 2, Geth Sub forms that appear are including Armature (possesses machine guns, heavy armor, shield), Hopper (uses sniper beam and light shield), Shock Trooper (armed with Pulse Rifle, Barriers, Carnage, and shield recharge capability), Sniper (armed with Sniper Rigles and Shielding ), Juggernaut (armed with Pulse Rifles, heavy shield, and distortion rocket), Destroyer (armed with Pulse Shotgun and strong shield), Dropship (standard troop transport), Colossus (armed with machine gun and strong armor and strong shield).
So, seeing that in both Mass Effect and Mass Effect 2 the Geth are holding such a wide role, it is no wonder that in Mass Effect 3, the role even bigger, with wider variety of sub-form that appear. Here is the whole list of sub-form of Geth that appear in Mass Effect 3:
Hunter (Also Appear in Mass Effect 2); it is a stealth troops that equipped with shotguns and shield. It is called hunter as it posses tactical cloaking device that allow them to attack in stealth
Trooper (Also Appear in Mass Effect and Mass Effect 2); a standard troop that armed with standard weapon and shield such as Pulse Rifles and Barriers. However, the main capability of Trooper is its capability to recharge shield using the Shield Boost ability
Rocket Trooper (Also Appear in Mass Effect and Mass Effect 2); similar state with Trooper but only replace the Shield Boost ability with the ability to launch scram and distortion rockets
Prime (Also Appear in Mass Effect and Mass Effect 2); elite unit that equipped with Pulse Rifles and very strong shield. Prime also has the ability to launch distortion rockets. Other main skill of Prime is it able to improves the combat skill of any nearby friendly unit as well as able to do radar jamming.
Pyro (new sub form that announced in Mass Effect 3); a new unit that considered as extremely dangerous for close combat, thanks to the flamethrower equipped. Also, this unit is very hard to taken down as it is coming with heavy shield and armor. Even so, this new Geth unit is very slow and quite harmless on ranged combat.