The Gluteal Sulcus
The gluteal sulcus AKA horizontal gluteal crease AKA gluteal fold AKA butt crease, is an area of the body highlighted by a noticeable horizontal crease. It is formed by the inferior aspect of the buttocks and the posterior upper leg. The groove which marks the lower limit of the buttock
The gluteal crease does not correspond to the lower edge of the gluteus maximus muscle. The glutes actually crosses the fold obliquely. The skin is firmly attached to the deep fascia by fibrous strands. It is formed during extension of the hip.
It is without a doubt one of the major defining features of the buttocks, though not everyone has it. When I see it (on a member of the female side of the human species) I just want to grab a spoon and get started on the dessert course.
Work It Baby!
If you want tight buns you need to get into some exercising! Pelvic floor pushes will do the trick, squats will blast 'em, I saw the ab circle pro on an informercial and there was one exercise that wasn't all about the abs that I think would really get that tush toned up.
Spot Reduction aka Targeted Fat Reduction
There is a myth where people believe that training and exercising a particular body part will specifically shed the fat on that part.
The fact is that you can not reduce fat in one area of the body to the exclusion of others. For example, doing sit-ups is not the best way to reduce belly fat. Sure, it will tone the muscle, but exercise helps reduce overall body fat.
The same applies to having a fat ass. You can blast those glutes, and it will firm up the muscle UNDER the fat. It will improve the shape and size of the MUSCLE. But fat is fat and if that's the only exercise you're doing, your probably not burning off enough calories to make a dent on the fat cells.
It is believed that fat is burned from areas of the body in pre-determined patterns (to do with your genetics as well as your sex) and is has no relationship to the muscles being exercised. For women, thigh and buttock fat is generally the last to be burned