When you're trying to lose weight, there are bookshelves, websites and TV shows full of people trying to sell you a dream. They say that you can lose weight quickly and easily, without any hard work, because of their new discovery. If a few people try it and it works for them, the press report it, and before anyone's had time to investigate the health implications or the long-term effects, we've got a fad diet on our hands.
Fad diets, basically, are diets that attempt to help you lose weight by making drastic changes to what you eat or some other element of your lifestyle, often telling you to only eat one thing or to avoid certain kinds of food completely. Now, I'm not saying that they can't help you lose weight, as many of them can - people on Atkins-like diets, for example, really do lose weight very quickly. But what people don't think about is the long-term.
So you've lost all those pounds on a fad diet, great. What next? You have a choice: either go back to your normal diet, or keep eating in a completely crazy way. If you don't want to keep on eating lettuce (or whatever) for the rest of your life, then you're stuck, and you'll just have to go back to your bad old ways - and a few months down the line, all those pounds have come back, and you go on the next fad diet. This is known as yo-yo dieting, and is even unhealthier than just being fat, as it increases your risk of heart disease and all sorts of other conditions besides.
In the end, if you want to lose weight, you should work out a diet that works for you - it doesn't have to completely traditional; as long as it's something you can stick to for the rest of your life.