They make someone urinate more frequently because they flush water and salts out of the body. Diurectics are commonly prescribed for people with kidney or heart disorders, or to treat high blood pressure.
Prescribed diuretics such as Hydrochlorothiazide and Amiloride are attractive to athletes who compete in sports where weight limits are imposed such as boxing, MMA and wrestling.
Banned under the Olympic Anti Doping Code, prescription diuretics cause people to lose excessive amounts of fluid thereby causing quick and temporary weight loss enabling users to effectively meet a weight that's actually lower than their muscle mass and weight would suggest.
As a result, a fighter who takes diuretics to make weight might weigh the same as his opponent but have a big - and unfair- size advantage because he has the body and muscle mass of a larger man. In addition, diuretics often help mask the by-products of other illegal, more serious drugs. So although they cannot be used to enhance performance themselves, they could hide the presence of other drugs which do.
As for health risks, unnecessary prescription diuretics consumption may lead to dehydration and potassium deficiency. And if not properly used, their side-effects can lead to kidney disease and heart failure.
This is not the opinion of a medical professional or someone even close. Always consult a physician before taking or discontinuing diuretics.