A stye is a small, painful lump that forms on the eyelid, usually caused by a bacterial infection in the oil glands. It appears red and swollen, resembling a pimple, and can make the eye feel tender or irritated. Styes can occur on the outer or inner part of the eyelid. Most styes heal on their own within a week or so. Applying warm compresses can help relieve discomfort and promote drainage. While styes are generally harmless, persistent or severe cases may require medical attention from an eye care professional, such as an ophthalmologist.