Poker games are played using 3 common betting limit strutures:
Bets and raises are made in fixed increments. In most poker variants including Holdem and Stud, the first 2 betting rounds use a small increment or bet size and later betting rounds use a large increment, usually double the small bet size. Stud games, split pot games, and most draw games use a fixed limit betting structure.
Bets and raises can be of any size with a minimum of the big blind amount. Any raise must be at least the size of the previous raise amount. For example, if Player A posts a big blind of 8 and Player B raises this to 20, a player wishing to raise this bet must make it at least 12 more (32 total). Games that commonly use this betting limit are No Limit Holdem and 2-7 Single Draw.
Bets and raises can be of any size between a minimum of the big blind amount and a maximum of the current size of the pot. As with no limit, any raise must be at least the size of the previous raise amount. The main difference is the limit on the maximum amount of a bet or raise. For example, if Player A posts a small blind of 4 and Player B posts the big blind of 8, Player C wishing to raise the maximum can raise it to 28 total. This is calculated as 8 to call making a pot of 20, and then betting the size of the pot (20) to give a total bet size of 28. Games that commonly use this betting limit are Pot Limit Holdem and Pot Limit Omaha.