Servitor
In the backstory of Warhammer 40,000, the "Men of Iron" (robots) rebelled against humanity, leading the Emperor of Mankind to place a ban on "Abominable Intelligence" (Artificial Intelligence). Unfortunately, robots are good for dangerous and/or dull tasks, and banning "the soulless sentience" didn't make those tasks go away.
Making a Servitor
Fortunately, the Imperium of Man has an extremely abundant supply of human beings, so the Adeptus Mechanicus takes unfortunate humans and converts them into Servitors: physically augmented and mentally stunted cyborgs. Officially, Servitors are taken from vat grown clone-stock or criminals too heinous to be placed in prisons. This can range from serial killer-rapists to people who steal from an Ecclesiarchy donation box.
Games Workshop actually quite consistent about the fact that most Servitors are generally blank-slates that are not even considered people and exist only to fulfil their assigned tasks. Vat-grown servitors are simply considered property as it would be Heresy to think of them as real people, whilst former human-servitors are generally mind-wiped unless they were created for a very specific reason, scrubbing away their former personalities and leaving nothing left to relate to.
However, what is not consistent is how much is actually left behind of the mind, as in some novels Servitors are fully capable of sustaining conversations and feeling simple emotions like panic, whilst other stories portray them as absolutely mindless to the point of stupidity; stubbornly performing their tasks even after years of irrelevance or during situations which are extremely not helpful.
So generally it boils down to whatever purpose they are designed for.
Types of Servitor
They come in all shapes and sizes, for performing tasks ranging from guarding tech-priests to stenography. Naturally, there are also servitors designed for use as soldiers as well.
- Technical Servitors - are the most common servitors; often referred to as mono-task, which have been cybernetically upgraded in a particular way to perform a single task, is usually a monotonous one where things could not go very wrong. They also are usually the ones who follow Tech-Priests or Techmarines around, holding tools and lifting heavy loads (like tanks) and generally being helpful on command. Mono-task servitors are most likely to get confused without direction and have a tendency to "mindlock" when without supervision.
- Combat Servitors - funnily enough are mono-task servitors kitted out exclusively for combat. Point and click servitors that are equipped with an array of offensive weapons either in close combat or are fitted with heavy weapons for long range.
- Note that combat servitors are still very different from the Skitarii though both are highly augmented, the Skitarii still have their personalities and free will.
- Tech Thralls/Adsecularis - a variant kind of combat cyborg, though considered even lower than other types of combat servitors. They have not had their personalities extinguished by mind-wiping yet but the amount of augmentation leaves them without free will; forced to charge at the enemy on command and are presumably shitting bricks while trapped inside their own bodies. Without the proper programming they aren't actually any good at fighting, yet are considered expendable troopers whose parts can be re-used later.
- Serfitor - Like the Adsecularis, a Serfitor can be mistaken for a servitor at first glance, though they have not had their minds wiped and have their bodies controlled by the machine. The reason for this subtle difference to normal servitors is that sometimes the crimes are not grave enough to warrant a permanent sentence, therefore once the period of servitude is complete they may attempt to return to their original lives. But now with a set of crappy augmetics that have left them permanently disfigured.
- Cenobyte Servitors - found amongst the Black Templars. Grimaldus has a set of them carry around relics.
- Arco-Flagellants - Prisoners guilty of crimes against the Ecclesiarchy have had no mind-wiping at all so they can suffer their punishment. Like Adsecularis, they are not strictly Servitors, but most of their bodies are cut-off and replaced with vicious weapons and their faces are enclosed with helmets that show them nothing but harmonious, serene, utopian visions, until a Priest or Inquisitor switches them to kill mode which changes the channel to torture porn and pumps them full of drugs, sending them on a murder spree.
- Cherubim - even vat-grown children (and the occasional real human baby) can be made into servitors and they are light enough to install anti-grav to let them fly. Though are generally considered vanity items, since any actual job they could perform can be equally performed by any other servitor.
- Murder Servitors - a specialist class of combat servitor, very much in appearance and function to Arco-Flagellants, though with with absolutely no personality what so ever as there is no crime to be punished for. Generally switched on as a last resort and then left to seek and destroy things (ie: everything in sight) at their own direction.
- Vigilies Pattern Servitors - Combat servitors on steroids, seriously, they are as big and as strong as Astartes, having been designed to take out individual targets one-on-one, armed with a power weapon and storm shield as well as an array of integrated weapons.