There is a well known bonus type of online slots: the free spins. It's also named as Free Spins Bonus Game and played on the slots reels.
You can receive this bonus in several events. First, free spins will begin automatically if you hit a combination of bonus symbols. The second variant is a connection of two bonus games: before free spins will begin you choose random amount of free spins on the additional screen. Thirdly, free spins can be as one of prizes in other bonus game (as a rule, this game on the second screen).
Free spins are frequently combined with multiplier of winnings. It is possible also, you can receive additional free spins during bonus free spins.
In general outline, bonus free spins are most attractive to players as prizes are increased repeatedly.