What are styes and chalazia?
Styes and chalazia are minor problems that can develop in the tiny oil (sebaceous) glands of the upper or lower eyelids.

Stye (or external hordeolum): A stye is an infection in the tiny oil glands along the edge of the eyelid that surround the base of an eyelash. It begins as a red, tender bump, and usually comes to a head in about 3 days. The stye then breaks open and drains and heals in about a week. See an illustration of a stye.
Internal hordeolum: An internal hordeolum, like a stye, is an infection. However, it infects the oil glands deeper inside the eyelid. Hordeola are usually larger and last longer than styes.
Chalazion: A chalazion is an inflammation of the deep oil glands inside the eyelid. It is not an infection. Chalazia typically grow slowly over 2 to 3 weeks, but usually do not cause pain. See an illustration of a chalazion
What causes a stye or chalazion?
The cause of a stye is often unknown, but it may develop from:

Rubbing the eyes, which irritates the oil glands. Bacteria that are present on your hand may cause the infection.
Using mascara, eyeliner, or other eye products, which can irritate the eye. If the products are contaminated with bacteria, an infection may develop.
A chalazion may develop from an internal hordeolum that does not drain and heal. Both internal hordeola and chalazia may develop when the oil gland's pore becomes blocked. This prevents the pore from draining, and it becomes inflamed.

A problem that can occur along with, or be mistaken for, a stye is an inflammation or infection of the eyelid (blepharitis).

What are the symptoms of a stye or chalazion?
A stye usually starts as a sensitive, red, swollen area on the edge of the eyelid at the base of an eyelash. This turns into a bump similar to a small pimple, which usually bursts open within about 3 days. As the stye grows, the eye may water and the eyelid may be painful. After it breaks open, the stye heals and goes away.

An internal hordeolum has the same symptoms as a stye, but it grows deeper inside the eyelid.

A chalazion grows more slowly, deeper inside the eyelid than a stye. Though it usually does not cause pain, a chalazion may last for several months. It may form a firm lump under the skin of the eyelid, and the inflammation and swelling may spread to the area surrounding the eye.

How is a stye or chalazion diagnosed?
It is often hard to tell the difference between styes and chalazia. Both are diagnosed by close visual examination of the eyelid. If there is a hard bump inside the eyelid, it is usually diagnosed as a chalazion.

How are styes and chalazia treated?
Although painful, a stye rarely causes serious problems. Home treatment including applying warm, wet compresses and using nonprescription eye ointments, solutions, or eyedrops's usually all that is needed to treat a stye. Once you've had a stye, you are more prone to having styes in the future.

Treatment for an internal hordeolum or chalazion is usually the same. However, if a chalazion becomes big enough that it interferes with vision, additional treatment may be needed.