A stye is a bump on your eyelid caused by a bacterial infection. A stye can form inside the eyelid (internal stye) or outside the eyelid (external stye). An internal stye may be caused by an infected oil-producing gland inside your eyelid. An external stye may be caused by an infection at the base of your eyelash (hair follicle).
Styes are very common. Anyone can get them at any age. They usually occur in just one eye, but you may have more than one in either eye.