Most online casinos offer slot machines that features a bonus. Bonus games are won by getting 3 or 5 bonus symbols, then on a second screen you will get to play the bonus game. In the bonus game you have a chance to win extra cash, free spins, or both.
The most common types of bonus games found in video slots are 'wheel of fortune', 'free spins', 'wild symbols', 'pick a box' and more. There are the slots bonus features which will be triggered on the reels or the bonus round can be activated on the second screen of slot game.
Most slots found at online casinos will have slots that feature multiple combinations of the games mentioned.
To activate bonus round on the online slot you must hit three or more bonus symbols (event should take place), or bonus feature will be triggered randomly.