The main difference between a stye and a chalazion is that a stye is an acute infectious process, and a chalazion is a chronic non-infectious process. Usually, a stye is the cause of the chalazion, although this is not always the case.
A chalazion occurs when the fat gland of the eyelid becomes clogged, a swollen lump in the eyelid.
A chalazion may begin as an internal hordeolum. At first, you may not know it is a chalazion because you may not feel pain.
However, as it grows, the eyelid may become red, swollen, and tender to the touch.