A stye or hordeolum is a boil or pimple on the skin of your eyelid. It usually happens when there is an infection in the root of the eyelash or a meibomian gland. A meibomian gland is a gland in your skin that produces an oily material which helps to moisten and protect your eyes  by preventing the water layer on your eye surface from evaporating. Symptoms of a stye are redness, pain and swelling on your eyelid. You may see white, pus-filled spot with a head on it close to the edge of your eyelid and it may be very tender as well. When left untreated it can become a chalazion. Once the initial inflammation has gone, you may be left with a bump that is painless on the underside of your eyelid. The most common cause of a stye is a bacterial infection. It is estimated that close to 90%  of styes are caused by staphylococcal bacteris, which is usually found  on your skin and inside your nose and mouth. Commonly  associated with styes is a process called blepharitis.