A bet is a declaration that either:

a) "I have the best hand and I'll wager money on it"  

b) "You have a poor hand, and you will fold if you are forced to wager on it".
 

Typically, players are supposed to bet when they have a good hand. Players who don't have good hands are supposed to fold. Of course, if it was this simple, there would be no need for this page. You might as well wager on Tic-Tac-Toe. Most players play contrary to this idea, attempting to be a cunning or deceptive player. Don't fall into this trap when you are just learning to play.

Your betting strategy should be built upon this simple idea, but you must know when to stray and bet in situations when you otherwise wouldn't. Here are some situations you should start looking at to improve your game:
 

Example one: Blind-stealing

When you are in the dealer's position, and only you and the blinds are remaining in the game, a raise is often called "blind-stealing". This is because the blinds may fold, whereas if you didn't raise but simply called, the blinds would simply check. Its a good way to make a buck or two, but will never make you rich. Its more of a way to end the game fast and have a new hand dealt with more players (and more money).


Example two: The steal-raise

If you are last to act and all players have checked to you, betting to simply limit the number of players or take the pot is called a steal-raise. Don't use this exclusively, as better players will be onto you quickly and begin check-raising against your (most likely) poor hand. It is good to use a steal raise when you have an excellent drawing hand such as a nut flush draw. Players will tend to "check to the raiser". If you draw to your hand, you now have a larger pot to win. If you don't, you can always check, and hope the fifth card makes your hand.
 

Example three: The check-raise

Check raising is checking to your opponent, with the intention of luring them to bet, so that you can raise them back. Your intention is to lure them into a false sense of security so that you can raise them and increase the pot (remember, after one bet is committed, its more likely they'll commit to two).

Example four: The opener

This reckless move is often done by people who bluff. It is when the person first to act raises, making all other players call two bets at once. Its inteniton is to limit the number of players. Basically, this move amounts to a backwards steal-raise. The effect will almost certainly cause many players to fold, but the ones remaining will either be equally aggressive or truly have a great hand. This is also known as betting for information. This tactic is best used with few players in on the hand.
 

Example five: Squeezing

Squeezing is a tactic only used in a short-handed game. It's betting when you have a good hand currently, and you suspect another player or players may be on a draw. For example, you have top pair with the best kicker. Chances are they won't make their draw (be it a straight or a flush draw, etc). Your goal is to limit their pot odds.

The above examples are just really examples of bets. If you are playing no limit or pot limit holdem, the whole thinking behind betting becomes radically different.