It is a common problem that most people suffer from occasionally and even though it is just an inflammation, a stye is extremely painful. By definition, a stye is an acute inflammation of the eyelid gland called the Zeiss gland. A stye is caused due to bacterial infections. When a stye develops, it causes a localised, painful, hard swelling near the margin of the eyelid and its symptoms include acute inflammation and redness of the overlying skin.