Mixed games refer to poker games where more than a single poker variant is played. For instance, HOSE is considered a mixed game where players play Texas Hold'em (H), Omaha/8 (O), Seven-card stud (S) and Seven-card stud eight-or-better (E).
Mixed games can consist of whatever games the players agree to and which the card room agrees to spread. Usually the games rotate as new dealers are brought into the game or after a set number of hands, such as a single table orbit.
These types of games are usually considered to be more difficult for players because a specialist in one type of game will be at a disadvantage in her non-specialties. This is why you'll often seen mixed games played at the very highest limits to equalize any perceived advantages.