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 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.