Bonus machines: These are slots which have bonus games. The way the bonus is activated depends on the machine, but you'll normally have to get a number of symbols on the winline over a number of spins. Then you'll get the bonus game, which should normally give you a decent prize. It is best to play these games when you can play for a while without interuption as when you leave the game any progress you have made getting toward the bonus will be lost.