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.

Cysts of Oral Cavity (Click the Image to Enlarge)
Classification of Cysts of Oral Cavity:
Cysts of oral cavity can be classified as shown below:
(A.)EPITHELIAL :
(B.)NON EPITHELIAL :
(C.)CYST ASSOCIATED WITH MAXILLARY ANTRUM :