A stye (also known as a hordeolum) is an inflamed/infected tear gland on the upper or lower eyelid which causes pain, redness and swelling. It typically doesn't affect vision unless the eyelid is swollen to the point where it obscures vision.
Styes are caused by bacteria very common in the body and isn't considered contagious except through direct contact. Treatment is usually a mild antibiotic ointment a 2-3 times per day and warm compresses twice per day for a few days. After this time it will come to a head and drain on its own. Never attempt to drain or "pop" a stye as it can cause serious damage to the eyelids and surrounding tissue and can occasionally cause the infection to go deeper into the tissues. Recurrent styes can indicate chronically clogged tear glands and lid scrubs can greatly aid in reducing the frequency of styes.
*Image found here