The stye is an infection of the implantation area of an eyelash called the "pilosebaceous follicle". It presents itself as a small, red, hot and painful bulge the size of a grain of barley located on the edge of the eyelid and centred on one eyelash. It appears within a day or two then when it ripens, it forms a pocket of visible white-yellowish pus on the surface of the stye. Also, it exists other symptoms.
The pocket of pus may most often be on the edge of the eyelid or on the skin (an external stye), but it can also happen that it develops toward the inside of the eyelid, in contact with the eye (an internal stye).