A stye (hordelum) is a lump on the inner or outer surface of your lower eyelid. The bump is usually small, painful, and pus-filled. It might look like a pimple. A mild stye might turn into a chalazion, which is a larger swelling/infection. Styes are infections that affect the glands in your eye and the hair follicles of your eyelashes. They are most usually caused by the staphylococcal bacterium. As these glands are responsible for keeping your eyes suitably moisturised, a stye can cause stinging and red eyes. A clogged meibomian gland, which produces a chemical that helps keep your eye moist, could be the cause of a stye on the inside of your eye. This gland can get infected, causing swelling in your lower eyelid.