The following article was sourced from a Wikipedia page at the following address: http://en.wikipedia.org/wiki/Lip_liner
Lip liner, also known as lip pencil, is a cosmetic product. It is intended to fill uneven areas on the outer edges of the lips after applying lipstick, therefore giving a smooth shape. It is also used to outline the lips, keeping lipstick inside the lip area and preventing it from "bleeding", therefore creating a bigger contrast and making the lips stand out more. The product is usually sold in a retractable tube or pencil form. Lip liner is usually available in the same range of colours as lipsticks: e.g., reds, pinks, browns, plums, etc. Lip liner also comes in invisible, for giving the illusion of smooth lips without adding or affecting colour.
To read more about lip liner, please click on the following link: http://en.wikipedia.org/wiki/Lip_liner