When people talk about ways to lose weight fast, they are usually thinking of a 'crash diet'. 'Crash' diets rely upon a drastic cutting back of calories, in the mistaken belief that the less you eat, the faster you lose weight.
Unfortunately, the weight lost on a crash diet in the first week or so is mostly only water, not real fat. In fact, most of the weight loss on any diet in the first few days is water, and many people find they lose between five and ten pounds in the first week on a new diet.
Another problem with the drastic reduction of calories of a crash diet is that weight loss slows, or even comes to a halt in a 'diet plateau', as your body soon realises it needs to conserve itself in the face of the apparent 'famine'. It does this by decreasing your metabolic rate, thereby making it even harder for you to lose any more weight. Avoiding this depressed metabolic rate or 'starvation mode' trap is the reason why healthy diets caution that weight should be lost gradually, not fast.
So miracle diets which claim to 'lose five pounds in ten days' or 'lose ten pounds in a fortnight' may well cause you to lose this amount on the scale. But unless you continue with the drastically reduced calorie intake for ever, the weight is likely to come back straight away.
Moreover, your crash diet may have done you more harm than good because these 'lose weight quick' diets are also often lacking in protein. A certain amount of protein is essential for the body and it cannot be stored for days or weeks later like carbohydrates and fats can. So if you don't eat enough protein every day, your body will take what it needs from your lean muscle. This is bad news as far as losing weight is concerned, as losing muscle reduces your metabolic rate even further.
This is a very complex topic which cannot really be explained properly in such a short summary. The book "Why Can't I Lose Weight" explains more of how it all works and how to avoid falling into the depressed metabolic rate trap.