like ronnoc10 said, there is a breakdown between concept artists for landscape, characters, etc. However, some artists do concepts for everything, so sometimes, they just take the general label and draw what the leads and directors tell them to draw.
However, some companies have the funding to have truly dedicated artists, some purely for landscape, some purely for characters, both main and NPC's (which is usually where most effort is spent). A good example of separation is the concept art for Fable 2. The characters have a unique look, one artists work, and the landscapes another, a good separation of what many artists who do what their good at can produce.