Primary brain tumors originate in the brain and occur as the result of abnormal changes to brain cells, known as mutations. As the cells mutate, they grow and multiply uncontrollably, forming a mass, or tumor. Cancers that have spread (metastasized) to the brain from other locations in the body are known as brain metastases. Brain metastases can be a single tumor or multiple tumors.
References: Cancer Treatment Centers of America