They make a wide variety now but originally it was a type of control top for your waist, to prevent muffin tops I guess. SPANX is the brand name. But now it's expanded to everything from thighs and legs to your booty to all the way up to your chest. For the average woman it's special occasion wear to get extra coverage/support and prevent panty lines in thin material dresses/outfits. It's not just for fat people, a lot of celebrities wear it. Off the top of my head I think Sandra Bullock had a scene in a movie about it.