Five opponent types: Station (loose-passive), Nit (tight-passive), LAG (loose-aggressive), Maniac (very loose-aggressive) and TAG (tight-aggressive).
A player who doesn't fold is a Station, a tight player a Nit, an aggressive player a LAG, an extremely aggressive one a Maniac, and a standard player a TAG. As a rule of thumb, VPIP 40% or more with raises on half of those hands or fewer is a Station, VPIP 45% or more with raises on 60% or more is a Maniac, VPIP 25% or more with raises on three quarters or more is a LAG, VPIP 15% or less is a Nit, and anything else is a TAG. Example: 4 hands played out of 10 with 1 raise is a Station. Each of the 5 opponents in play has one of these types, which stays until you reseat.