A brain cyst is not a tumor but is a tumor-like sphere filled with fluid, notes the American Brain Tumor Association. Besides fluid, a cyst may be filled with blood, tissue or tumor cells. Generally noncancerous, brain cysts can interfere with vital functions due to location in the brain.
There are many different types of brain cysts, notes the American Brain Tumor Association. Arachnoid cysts occur in adults and children, and are found between the layers covering the brain. Colloid cysts most often occur in adults and are filled with a type of gel-like substance. Dermoid cysts normally occur in children under the age of 10, and epidermoid cysts are normally found in middle-aged adults.