Fixed Limit: This is probably the most common playing style in terms of the number of games that use it, although recently it has been eclipsed by the no limit betting structure in the casinos of North America and Europe. In any case, fixed limit is the recommended starting point for beginners because the stack swings are the easiest to deal with in the limit betting structure. If a game is defined as fixed limit $2/$4, then it means that $2 is the small bet and $4 is the big bet. In the first half of the betting rounds, betting and raising occur in increments of the small bet. In the remaining rounds, they are done in increments of the big bet.
Spread Limit: Spread limit used to be one of the most popular betting structures around, but recently it has fallen steadily in terms of the number of people that use it. Nowadays, it is not very popular at all outside of home games of people that have been playing poker since long before the poker boom occurred. In any case, the spread limit works in a way similar to the fixed limit, but provides a bit more flexibility for the players. If a game is defined as being a $2-$4 spread limit game, then what it means is that players must always make a bet between $2 and $4 in each betting round. Players must raise the amount of the largest bet made at least, although they are free to raise up to $4 if they wish. No bets or raises may be larger than the higher end of the spread limit, or in the case of this example, $4.
Pot Limit: This betting style is very popular with some of the board games such as Hold 'Em and Omaha, but at the same time is not very popular with stud games. In this game, the only thing that is limiting the amount a person can bet or raise is the size of the pot. As the pot grows, the amount a person can bet gets bigger and therefore as the game progresses you might find that the pot size is greater than your stack size in which case you can go all in.
No Limit: The Cadillac of betting styles, there are no restrictions on how a player may bet in no limit. At any point during the game they can go all in and force their opponent to either commit the same amount of chips to call or else to fold. The biggest bluffs take place during no limit games, because in no limit you can force your opponents to make decisions for all of their chips on a regular basis.