Calling raises before the flop is a huge mistake that most unaware players make time and time again. In fact, calling a raise 'cold' (calling two bets before it has been your turn) is such a huge money-loser that it warrants an entire lesson in and of itself. As outlined in Lesson #2, there are very few hands that realize a long-term profit in hold 'em. Following this logic, there are even fewer that show a profit after someone has raised.

The reasoning behind this is easy to understand. There are plenty of players who will raise with a wide variety of hands (if you have played on-line poker before you are likely to have noticed this); however, the 'typical' player, when raising before the flop, will often hold either AA, KK, QQ, JJ, AK (suited or offsuit) or AQ (usually suited). Given this small range of hands, you only want to play those hands which play well, and are not 'dominated', by the hands within your opponent's 'raising range'.

Pretend you have KQ offsuit, for example. Usually, this is an okay hand to play. But it plays horribly against the typical player's raising hands. If your opponent has AA, you would need to catch both a queen and a king to win.  If your opponent has KK, you would need to catch two queens.  When you're up against either AK or AQ, we're back to the example illustrated at the beginning of Lesson #2 with the A T vs. the A J. To make the situation worse, if the flop gives you the second best hand you'll often end up paying through the nose, since a) you'll think you have the best hand when in fact you don't, and b) your hand will usually have less than a 20% chance of improving to a winner.  As you can see, a lot of things have to go right for you to drag the pot when you're up against a player with a high-quality starting hand. For obvious reasons, this is not the spot you want to be in.

If you're in a 'typical' low limit game, and an early-position player raises and you call, you should have one of three hands; either AQ suited, AJ suited, or KQ suited. You could even consider folding these hands if you know your opponent is a good player. With any other hand you should fold or reraise (see Lesson 2, Situation3 for when to reraise)). The exception to this rule occurs when you can anticipate a lot of other players calling the raise; then you could mix in some of the small pocket pairs and larger 'suited connectors' (suited cards that are next to each other in rank) and hope to catch a flop. Otherwise, you're done with the hand. 

Note: it is rarely correct to fold if you've already called once, and an opponent AFTER you raises. What we're discussing here is the practice of calling a raise 'cold', i.e, calling two bets before you've put any money into the pot.