WARNING: The video in this post is extremely graphic and anyone who has even a slightly weak stomach should consider whether to watch or not.
What is a cyst?
A cyst is a closed sac, having a distinct membrane and division compared with the nearby tissue. Hence, it is a cluster of cells that have grouped together to form a sac (not unlike the manner in which water molecules group together, forming a bubble); however, the distinguishing aspect of a cyst is that the cells forming the “shell” of such a sac are distinctly abnormal (in both appearance and behaviour) when compared with all surrounding cells for that given location.
It may contain air, fluids, or semi-solid material. A collection of pus is called an abscess, not a cyst. Once formed, a cyst may sometimes resolve on its own. When a cyst fails to resolve it may need to be removed by surgery but this will depend on what type of cyst it is and where in the body it has formed. Cancer-related cysts are formed as a defense mechanism for the body, following the development of mutations that lead to an uncontrolled cellular division. Once that mutation has occurred, the affected cells divide incessantly (and become known as cancerous), forming a tumour.
The body encapsulates those cells to try and prevent them from continuing their division and try and contain the tumour, which becomes known as a cyst. That said, the cancerous cells can still mutate further and gain the ability to form their own blood vessels, from which they received nourishment before being contained. Once that happens, the capsule becomes useless and the tumour can go from benign to a cancer.