A sty, stye or sties if there are more than one of them is a bacterial infection of a eyelash follicle. The infection causes a lump to form in or outside of the eyelid. The lump will grow for about a week and usually subsides, somtimes bursting as it heals.
Sties should not be squeezed like pimples, as squeezing can spread the infection and make matters worse. If you have had a sty once then you are more likely for them to recur.
Doctors often recommend holding a warm, moist cloth against the affected eye to lessen the swelling. They also frequently prescribe antibiotics that attack the bacteria.