There are three basic betting structures in poker,
This is the most basic form of betting. You only have the choice of set betting amounts and there is a fixed amount of raises.
this refers to the maximum bet being the total held in the pot at any given time.
as the name suggests there is no limit to the amount you can bet, other than the amount of cash/chips you have at the table at the start of the hand. This is the most volatile kind of poker as mistakes could cost you a lot of money.