Computer and video games are generally categorized into genres. Due to a general lack of commonly agreed-upon genres or criteria for the definition of genres, classification of games are not always consistent or systematic and sometimes outright arbitrary between sources. Action games are perhaps the most basic of gaming genres, and certainly one of the broadest. Action-adventure games focus on exploration, and usually involve item gathering, simple puzzle solving, and combat. Computer role-playing games (CRPGs or simply RPGs) often place the player in a fantasy or science fiction setting and drive the gameplay via a prominent storyline.