Betting, Raising, Calling, Checking, and Folding

Each possible action in a betting round is done for strategic reasons.

Betting and Raising

Betting refers to the making the initial bet to start a round. Betting can be used to increase the money in the pot, or to force players out of the hand. It is an aggressive action used with strong cards, or cards you wish to represent as being strong.

Raising refers to increasing the bet size after someone else has bet. It is similar in intent to betting, yet is even more aggressive because someone has already shown strength first by betting.

Checking, Calling, and Check Raising

Checking refers to not making any action when there is no bet to you. You cannot check pre-flop since there are the blinds who bet before the cards were dealt. This is generally the most passive play, yet one you should use often with weak cards as it costs you nothing and everyone may check the round allowing you a chance to improve for free.

Check-raising however is among the most aggressive plays. That refers to checking with the intention of raising when someone bets. It is risky because everyone else could check and no money would go into the pot. This is generally done with very strong hands, and in fixed limit generally done on the turn where the betting size has increased.

Calling refers to equaling the current bet size. It is often used when you need future cards to improve, or have a moderate hand and believe your opponent is representing more than their actual strength by betting. It is a very passive play, and in situations where you can't decide between calling and raising it is generally better to raise. This is often thought of as giving people a chance to fold. In many cases (especially in no limit) either raising or folding is better than calling.

Folding

Folding refers to discarding your cards and not investing any more money in the pot. It is one of the most important actions in poker. Limiting your losses is every bit as important as raising to maximize your winnings, perhaps even more so for new players.

Playing on the River

In fixed limit, playing with a moderate hand on the river against a single opponent is different and happens often enough to be mentioned. In this case folding is a very risky play--you have to be absolutely sure your opponent has you beat. Folding to save a single bet could cost you an entire large pot, so calling is often recommended. Betting a moderate hand on the river is not recommended either. Better to check and call if your opponent bets. When he has a weak hand you win the same amount since he would likely have folded to your bet, if he has a strong hand you lose less since he was not able to raise you, and if he bluffs you win an extra bet.