Autocockers are basically pump guns that "AUTO"matically re"COCK". They are re-cocked by pneumatics at the front.
What makes them quite uniqe is the fact that they are closed bolt. Meaning when your pull the trigger a ball is already in the breech, so that is fired first, then the autococker recocks putting another ball in the breech ready for the next shot. This is all done by quite complex "timing", each componant that is involved has to be timed so it works at the right time to make the whole thing work.
They are a mass of moving parts (which makes them look pretty cool) but most of these parts can be replaced by aftermarket ones that other companies make, to help rate of fire or efficiency. Also the original body has quite a lot of waste in it, custom cockers (nick name for autocockers) replace what parts they can (sometimes) and usually carve (mill) a unique design into the body, making it look purrrrdy and make it lighter.
Cockers will be about the only non-electro you will see "pro" players use. Many players like them because they can get a very individual "Look" that is unique to them, and they can tune it to get what they want, either high rate of fire, better efficiency or better concistancy.