Slots are easy to play, can be large payers (in progressive games) and have great variety - so they are very popular in online casinos. A 'progressive' game means that a combination of jackpots are combined from different players in different casinos. Such machines pay out less in total money than an average slot machine but offer larger jackpot prizes in return. Expect to pay in a fair amount of money into such machines before you can compete for the large jackpots.