Having got to grips with Outs and Odds, it's time to move onto Implied Odds, which is nothing more than a natural extension of Pot Odds. Whereas Pot Odds take into account the money that is in a pot right now, Implied Odds are an estimation of how much might be in there later, if betting continues. This can have a very real impact on your decision to continue drawing to a hand or not.
A classic example of Implied Odds can be found in a game of Fixed Limit Texas Hold'em where a player is four to a flush after the flop has come down. In a game of $2/$4 Fixed Limit there might only be $10 or so in the pot, with three players still in the hand, when it comes to your turn to act. If you're being asked to pay $4 to stay in the hand then you're not really getting any decent pot odds at that stage.
However, you can see that your opponent has hit something and will continue to bet aggressively, and that the other player is also keen to stay in, perhaps looking to make a hand of his own. With that in mind you know the pot is going to grow and so can work out what it is likely to be by the time the river card comes along, which should help you decide whether or not you want to stay in the hand.
That's all there really is to implied odds. It's just a case of projecting ahead and estimating what is there to be won by the end of the hand, and not just what is there to be won at that moment. In Fixed-Limit games, where there are multiple players remaining in the betting, it is nearly always the right thing to stay in the hand if you have a lot of outs for a very strong hand. In Pot Limit and No Limit games you have always got to take into account that a very big bet, or even an all-in, may be just around the corner.
When you do start to make these implied odds calculations it is always best to err on the side of caution. A commonly made mistake is to over-estimate what is likely to be in the pot by the time the river card comes, with the effect that you make bets that are not really giving you the right value.