It uses the Unreal Tournament engine I believe.
I found it to be full of racial stereotyping with lots of toilet humour like being able to whip your thingy out in public and wee on people (aim for the mouth to make them puke), lots and lots of violence (I especially liked the sledgehammer used as a weapon).
A lot of people with think this thing is just sick sick sick, and it has quite rightly been banned in a lot of countries.
It's good for a quick fix of mayhem though. There is a complete version with extra weapons and fan made maps and mods called the Fudge Pack, but I found it prone to crashing a lot.