Most bumps on the eyelid are styes. A stye is an inflamed oil gland on the edge of your eyelid, where the lash meets the lid. It appears as a red, swollen bump that looks like a pimple. It is tender, especially to the touch.
A stye is caused by bacteria from the skin that get into the oil glands in the eyelids that provide lubrication to the tear film. Styes are similar to common acne pimples that occur elsewhere on the skin. You may have more than one stye at the same time.
Styes usually develop over a few days and may drain and heal on their own. A stye can become a chalazion -- this is when an inflamed oil gland becomes fully blocked. If a chalazion gets large enough, it can cause trouble with your vision.
If you have blepharitis (see eye redness), you are more likely to get styes.
Other possible eyelid bumps include:
Pasternak A, Irish B. Opthalmologic infections in primary care. Clin Fam Pract. 2004; 6(1):19.
Noble J. Textbook of Primary Care Medicine. 3rd ed. St. Louis, Mo: Mosby; 2001.