Lean weight includes the muscles, bones, tendons, ligaments, and water in the body.
Fat weight includes the fat stored in the fat cells and the organs of the body.
For good health, it is important to have a high proportion of lean weight compared to fat weight. Lean weight produces work and burns food calories. Fat weight is excess baggage for the body and this can be a detriment to health. The most accurate way to measure lean weight is to weigh a person underwater. This is done by scientists for research mainly on athletes.