A humanoid is any living being that is like a human. Generally, this will mean have a torso, two arms, two legs, and a head. A tail need not prevent something from being considered humanoid, nor need extra or missing parts, if the general correspondence to humanity is adequate for the current purposes of classification.
Humanoids generally may use weapons, armor, shields, and other items. Some may use magic. If the game allows for conversation, they will general talk (although the sophistication of conversations in roguelikes is generally quite low). It is not unusual for humanoids to appear in groups, act "intelligently" (by the standards of the particular game), and may show some rudimentary signs of social or religious organization.
Generally, most playable races will be humanoid, although some are not. Some common humanoids in roguelike games include elves, dwarfs, orcs, goblins, trolls, and halflings, although many others will be encountered. Technically, humans are humanoid, but they are rarely thought of as included in the term "humanoids".