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.