Could you explain weight loss to me?

I am curious. Everybody is talking about weight loss. Some of them talk about it like it is the most important thing in the world, especially women. Sometimes it is caused by some illness. But could somebody explain to me what the weight loss really is? How all that weight disappears?

Well, I suppose that like every other human being you are doing some things like walking, going to the job or in the school, dancing, jumping, swimming, playing etc. All this things require energy. If your body does not take enough energy from the food and liquids then it has to use fats from your body to make the energy. Also weight reduction can be caused by loss of fluids, muscles and bone mass.