A cyst is a closed sac, which has its own epithelial membrane, and which is enclosed within an organ. In the vast majority of cases, these cysts contain a liquid secreted by the epithelium and may become symptomatic depending on the size and tension created by the content in the organ in question (the classic example being that of a mammary cyst).
All organs of the body may contain cysts. In some cases, the content may be gaseous rather than fluid. Some cysts can become infected spontaneously or as a result of manipulation.
The Pseudo Cyst differs from the cyst in that no epithelium surrounds the liquid. Rather, the surrounding soft tissues hold the liquid in place (the classic example being that of the pseudo pancreatic cyst).