A cyst (soft "c", rhymes with "list") is a closed sac having a distinct membrane and develosion on the nearby tissue. They may contain air, fluids, or semi-solid material. A collection of pus is called an abscess, not a cyst. Once formed, the cyst will remain in the tissue permanently and can be removed by surgery and by taking medication that will dissolve the "sac".
A cyst may also be a sack that encloses an organism during a dormant period, such as in the case of certain parasites. This type of cyst may, for instance, protect a parasite from the churning acid of the stomach so it may pass through to the intestines unharmed where it can then break out. Cystic fibrosis is an example of a genetic disorder whereby cysts and fibrosis develop in the lungs.