A stye develops when a gland at the edge of the eyelid becomes infected.  They can develop on the inside or outside of the lid.  Styes can occur at any age and are not usually harmful to vision.  They are caused by staphylococcal bacteria which is often found in the nose. 

Symptoms

Symptoms include eyelid redness, tenderness, pain and swelling.  A small pimple-like bump may appear.  Sometimes the entire eyelid may swell.  The affected eye may be watery or have a foreign body sensation (feeling as if something is in the eye).

Treatment

Most styes resolve on their own within a few days.  You can encourage this process by applying hot compresses to the area for 10 to 15 minutes at a time three or four times a day.  A hard-boiled egg wrapped in a damp cloth will hold the heat longer than a cloth alone, and can be reheated and reused as long as it remains intact.  The warmth helps "draw out" the fluid inside the sty and can speed resolution.  Never try to "pop" a stye like a pimple.  If you have frequent styes, your eye doctor may prescribe an antibiotic ointment and/or an eyelid hygiene regimen.

Chalazion: another type of bump

Similar in appearance to a stye, a chalazion is an enlarged, blocked oil gland in the eyelid.  A chalazion is similar to a stye for the first few days, then turns into a painless, hard, round bump later on.  Most chalazia develop further away from the edge of the eyelid than do styes.

The same treatment speeds the healing of a chalazion, but the bump can linger for much longer.  If the chalazion remains after several weeks, your eye doctor may drain it or inject a medication into it to speed healing.