An orc is a humanoid foe, common to most fantasy roguelikes. Orcs may come in a wide variety, representing different tribes or subspecies (hill orcs, valley orcs, tunnel orcs) or specialists (orc captain, orc wizard, orc shaman). They frequently have access to ranged weapons and/or magic.
In games where monsters have an inventory, orcs will typically be equipped with a variety of weapons, armor, and possibly other items. If the game ascribes racial workmanship to items, orcs will normally carry "orcish" gear.
Orcs usually appear in groups and act aggressively. In a game which uses the concept of alignment, orcs will normally be evil or chaotic.
Orcs are well-liked by roguelike creators because they are excellent fantasy sword-fodder, evil and violent, but capable enough to appear through many different levels of the dungeon. They also can be represented in ASCII by the letter 'o', which doesn't have a lot of options.
Orcs are sometimes available as a playable race.