Generic role where actor often appears in a nonspeaking, nonsinging or nondancing capacity, usually in the background. May have one line in speaking role. Physical and skill requirements will vary from scene to scene. The pool attendees for example are a type of background actors with specific physical requirements. Even characters killed onscreen may be asked to return in a different form for another scene.
Copy and credit (i.e. no monetary) generally. Often on location perks are provided, and in some cases $10 - $20 total is given to cover gas.