The term FREEROLL is used to describe a poker tournament that has no entry fee.
The prize pool is actually a donation from the poker room (the house), which includes all fees. Sometimes a poker room will offer a freeroll tournament to its most frequent players.
A freeroll should not be confused with playing in real money tournaments; freeroll tournaments are free whereas a real money tournament requires a payment.
Freeroll tournaments are not exclusive to poker though... casinos often arrange freerolls in games such as slots, blackjack and video poker.