You put your body under undue stress for a short time. It doesn't know what hit it, so thinks there is a famine. As a result, it slows down the metabolism and you gain weight really fast when you return to eating normally. Crash diets usually involve eating something ridiculous, as Good soulette says, that you cannot possibly do for long, so when you go back to eating normally, you regain weight- and more. Worse, you end up craving the things you are depriving yourself of and as a result, eat too much when you finally give in.
If you want to lose weight, just eat 3 small meals a day- cereal, sandwich and dinner which is low carb and smaller than normal in quantity. No snacks, no extras. You will lose weight, but you can still basically eat normal food, and food that your body is telling you it needs. If you crave sugar, have an apple, or add some honey to porridge for breakfast. I've been doing this since Jan and have lost 2 stone, and don't feel deprived at all. x