There are all kinds of online slots. Slots can come in many themes, payout schemes and styles. Still, most slots are divided into classic slots and modern video slots. Classic slots are the simpler more old fashioned slots, similar to the ones you used to find at arcades.
This type of slots is still popular and this has given rise to retro style slots. These usually have three reels and one pay line. The gameplay is usually simpler. Thus Modern video slots on the other hand, often have five reels and any number of pay lines. They contain bonus features such as free spins, mini-games and more.