First, a regular scale tells you pretty much nothing at all. Let's say you're 5'5" and 130 pounds. You could be a hard core mountain biker, thin and lean, with a fantastic body shape and great strength. You could also be a person who watches TV all day and eats gallons of ice cream, and be very round. Both would have the same weight - but would be on far different ends of the health spectrum.
That's why it's critical to have a body fat scale. Body fat scales send a gentle electrical impulse through your feet. You don't feel the signal at all - it is completely undetectable to humans. The scale can detect it, though. Because fat has a different resistance to electrical signals than muscle does, the scale can tell how much fat is in your body based on the length of time the signal takes to go from one foot to the other.
How about other things in your body? Usually it's only fat vs non fat that people care about. The non-fat items in your body - bone, organs, etc. - are all the size they are for necessity. The only things you can really "change" are the muscle and fat - so you want to increase the muscle while you decrease the fat.
You might be interested to know that your body on average has 6 pounds of skin!
How about bones? On average, bones make up 20% of your total body weight. If you weigh 150 pounds, this means that you are 30 pounds of bone.
Now here's a tricky one. How much of your body is water? Most estimates are that 70% of your body mass is water. How can that be? It's because that water is in every part of you. A portion of your bone weight is water. Your fat is 50% water.
Never try to fast yourself to lose weight by not drinking water! You need that water in your bones and muscle, and water is critical to good metabolism. You have to get rid of the fat. Drink ample amounts of water every day, to keep your metabolism moving along smoothly.
Lisa Shea's Library of Low Carb Books
