Freeroll tournaments are extremely prevalent and equally popular in the world of online poker. Basically, a free roll means that you do not have to pay cash for the entry fee. Your ticket to the tournament has no monetary cost.
Many freeroll poker tournaments require some other method of qualification, usually player points (like frequent flyer miles on an airline) that can be redeemed for a seat at the freeroll. New online poker rooms use freeroll tournaments to attract players. They put up the prize money as a marketing cost, and allow the players to compete at no charge.
In poker, a freeroll can refer to any situation where a player has the chance to win a pot without risking any money to do so. But freeroll is generally defined as a free to enter poker tournament hosted and billed to an online poker room.