What is lost from the body when a person takes a diuretic for weight reduction?

Diuretics. What is lost in these cases is fluid. The body is mostly water so if you lose significant amounts of water your weight will go down, but not necessarily the fat mass.
Water. A diuretic causes the loss of water. It does not remove fat. It may cause rapid weight loss at first but will not continue to lose unless diet and exercise are part of the program.