Betting Limit Models

There are three types of betting models in Texas Hold'em.

Limit-hold 'em - Bets and raises during the first two rounds of betting (pre-flop and flop) must be equal to the big blind; this amount is called the small bet. In the next two rounds of betting (turn and river), bets and raises must be equal to twice the big blind; this is called the big bet.
No-limit Hold 'em - This is the variation that is played at the World Series of Poker and other Hold 'em tournaments. Players may bet or raise any amount over the minimum raise up to all the chips the player has on the table (called an all-in bet). If someone wishes to re-raise, they must raise at least the amount of the previous raise.
Pot-limit Hold 'em - The maximum raise is the current size of the pot.