There are two classifications of headaches, and cluster headaches are in a class referred to as primary headaches, and clusters are the most severe form of primary headaches.

Primary headaches occur on their own. Meaning that there is no other medical disease or problem that is causing the headache. This class of headache is divided into four main groups; migraine, tension, trigeminal autonomic cephalgias (for example, cluster headaches, paroxysmal hemicranias, and SUNCT), and a miscellaneous group.

Secondary headaches are caused by an existing illness or disease that affects the brain. The more serious examples include brain tumors, hemorrhagic and ischemic stroke, and head injury.

This kind of headache occurs in clusters, usually at the same time of day or night for several weeks. Clusters of attacks of pain strike one side of the head, for example behind or around one eye. The patient may even experience nausea and a migraine like aura. The intense pain generally peaks around 5 to 10 minutes from when the headache first begins, and then the pain continues at that peak intensity for up to three hours.