Orcs arrived as slaves of the Drow, and for the most part, they remain so to this day. Although they have been known to flesh out the troops of the Shadowlands, they are generally considered to be a cowardly race, and unwilling to commit to a battle unless victory is fairly certain.
By and large, the more dangerous opponents in the armies of the Dark are the half-orcs. These cross-bred slaves of orcish and human ancestry are smarter and more resourceful than their orcish parents, but not as independant as humans, and so make highly desireable slaves. They are frequently (but not always) incapable of breeding, however, and thus perpetuate the need for both parent-stock in the drow slave-pens.