Lip liner, also known as lip pencil, is a cosmetic

Cosmetics

Cosmetics are substances used to enhance the appearance or odor of the human body. Cosmetics include skin-care creams, lotions, powders, perfumes, lipsticks, fingernail and toe nail polish, eye and facial makeup, towelettes, permanent waves, colored contact lenses, hair colors, hair sprays and...

 product. It is intended to fill uneven areas on the outer edges of the lips after applying lipstick

Lipstick

Lipstick is a cosmetic product containing pigments, oils, waxes, and emollients that applies color, texture, and protection to the lips. Many varieties of lipstick are known. As with most other types of makeup, lipstick is typically, but not exclusively, worn by women...

, therefore giving a smooth shape. It is also used to outline the lips, keeping lipstick inside the lip area and preventing it from "bleeding". The product is usually sold in a retractable tube or pencil form. Lip liner is usually available in the same range of colors as lipsticks: e.g., reds, pinks, browns, plums, etc.