Orcs are the foot soldiers of evil, the creatures that make up the hordes of darkness that threaten to overwhelm the good peoples and turn the entire land into a region of little else than slaughter and depravity. Most Games Masters use them as the guardians of low-level dungeons, just a slight step above kobolds as a threat. Often they are simply portrayed as evil humans and are given little more thought than that.
Used availability for Matt Forbeck's The Slayer's Guide to Orcs