A sticker is a piece of printed plastic, plasticised paper, or paper, with adhesive backing which can be removed from its backing and applied to a surface. Some people use the term sticker and decal interchangeably but a decal uses water transfer.
LEGO uses a lot of stickers for parts where printing the part didn't seem to make sense or for designs where the sticker spans more than one part creating a STAMP. There is also some question about producing stickers being cheaper for the company than printing directly.
There is some purist/ collector controversy about whether you should or should not apply the stickers to a built set, since once applied, they typically cannot be removed without ruining them and, in the case of STAMP (STickers Across Multiple Pieces), joining multiple elements together more or less permanantly.