Buns are usually made from flour, sugar, milk, yeast and butter. Common varieties contain small fruit or nuts, are topped with icing or caramel, or filled with jam or cream. Some types of buns are filled with various meats.
A bun is normally made from dough that has been enriched with sugar and butter and sometimes egg. Without any of these the dough remains to be 'bread dough' rather than 'bun dough' and the resultant product will be called a roll, rather than a bun.