The point of a cleanse is essentially to "reset" your digestive and immune functions, and of course, to lose weight. Basically, it works because you're barely eating anything and you become "thinner" simply because you're low on water weight.
To be effective, you need the feed yourself the right kind of nutrients to flush out toxins from your body. To be safe, you shouldn't do a cleanse for longer than 2-3 days.
The most basic cleanse I know of is mixing lemon juice, maple syrup, and water. There are different varieties, but to me, these kinds of cleanses are more like fasting than healthy dieting.
Cleanses aren't meant as a long-term solution for weight loss. They're best used as a pre-cursor to starting a weight loss program or diet, as a general boost to your immune or digestive system, or in a pinch such as when you're trying to dry out and look cut right before a con.
If you're doing a cleanse, stick to the ingredients that you're told work (and that you can withstand for the time you spend fasting) - otherwise, it's not a cleanse but a protein meal.