Humans are a species of surface-dwelling humanoids. They are relatively sophisticated, tough, intelligent, and adaptive - which makes them very good generalist slaves. Still, they lack the specialized ablities of other slave races. They are best used as house servants, lovers,and leaders among slaves.
The main trouble with human slaves is that once a few of them get together they almost always start rebellions or try to escape. Thus most owners of humans keep them separated from eachother.
One very successfull Crossbreed is the Human-drow halfbreed which combines a more docile nature with the resilience of a human.
Because humans are capable (and sometimes even willing) to breed with just about anything they are somewhat popular among BreedingMistresses
Humans come in a variety of shapes and sizes and colours and they do not display anything like a unified culture or excel at anything, rather being fairly good at pretty much everything. Furthermore humans do not have a single god as their protector (Like Lolth for the Drow or Laduguer for the GrayDwarf s) but rather a swarm of deities whom all seem to compete or cooperate all willy-nilly.
It is commonly accepted that this is the cause for the variety of human looks and attitudes. What else could be expected by a race created by gods who can't agree with or on anything?