A 'cyst is a closed sac having a distinct membrane and division on the nearby tissue. It may contain air, fluids, or semi-solid material. A collection of pus is called an abscess, not a cyst. Once formed, a cyst could go away on its own or may have to be removed through surgery. Locations Acne cyst - Pseudocysts associated with cystic acne. Actually an inflammatory nodule with or without an associated epidermoid inclusion cyst. Arachnoid cyst (between the surface of the brain and the cranial… (More on Cysts)