The Legio Cybernetica is the robotics branch of the Adeptus Mechanicus. It is one of the oldest branches, with records stretching back to the very first days of the Imperium. http://wh40k.lexicanum.com/wiki/Legio_Cybernetica#.UgA4zqbn8iv
The Legio Cybernetica has produced many Robot designs over the centuries. Some were failures, such as the disastrous Castigator, a Robot so heavily armoured that it was slower than the troops it was designed to protect and support. Others proved more successful:
A light and agile Robot, the lightly-armoured Crusader is designed for quick, debilitating strikes. Specialized in anti-personnel combat, it is generally not fielded when heavy resistance or armour is expected, though it is a favorite of the Inquisition for hunting down heretics. The Crusader is armed with a power sword and twin lascannons, though its ranged weapons can be replaced with a heavy bolter and melta gun.
A siege Robot, the Colossus uses a Siege Hammer to pound down enemy fortifications, though it is equally useful squashing opponents. Other armaments include a boltgun and melta gun. Protected by a power field/synchroniser unit, its weaponry can be replaced with autocannons, heavy bolters, and power fists, allowing for a number of combinations.
The Cataphract is a general-purpose, heavily-armoured Robot, armed with a lascannon, boltgun and flamer and protected by a power field/synchroniser unit. Its versatility makes it popular Robot to deploy, although some commanders regard it as a jack-of-all-trades, master-of-none. It can substitute its weapons for an autocannon, Siege Hammer, heavy bolter and power fist.
Designed for anti-vehicle and siege work, the Castellan is an assault Robot useful for a wide range of battlefield roles. Its twin power fists and heavy bolter give it a dangerous punch, protected by a power field/synchroniser unit. It can subsitute its heavy bolter for a lascannon, melta gun or third power fist, while the standard fists can be replaced with a Siege Hammer, autocannons and flamer.
Protected by heavy armour and a power field/synchorniser unit, the Conqueror is particularly adept at taking out armoured vehicles and Dreadnoughts. Standard armament is a heavy bolter, autocannon and power fist, which can be swapped for lascannons, flamers, melta guns and bolter, as well as doubling up on any one.