A bun (also known as a bobtail) is a type of hairstyle, typically worn by women, where the hair is pulled back from the face, twisted or plaited and wrapped in a circular coil around itself, typically on the back of the head or neck. They can either be secured with a hairpiece, a hairnet and bobby pins. They may be tightly gathered or slightly messier and more informal. Buns, like ponytails, may cause headaches if worn too tight or up too high on the head.
Men are not often seen wearing them, but they can do so. This may be because they are using it in a practical way such as in sport (like samurais) or using it in a formal way.
Also known as a " ballet bun," this hairstyle is used by nearly all female ballet dancers and some male ballet dancers, with a few notable exceptions. Ballet dancers often use hairnets and bobby pins to make the bun as tight and neat as possible. This is especially important while turning, as loose buns may fall out and the bun has to look presentable.
Basically, bun hairstyles are just twisting the hair in such a way that it is coiled around itself.