A sticker is a type of label: a piece of printed paper, plastic, vinyl, or other material with temporary or permanent pressure sensitive adhesive on one side. It can be used for decoration or for functional purposes, depending on the situation.
Stickers can come in many different shapes and sizes and also vary widely in color and design. They are often adhered to items such as lunchboxes, paper, lockers, notebooks, walls, cars, windows, used as name tags, and so on.
The term "sticker price" refers to the historic practice of adhering a large sticker to the window of a new car listing its base price, options, shipping charges, etc. (from which a discount was often negotiated).