According to the Gale Encyclopedia of Neurological Disorders, epilepsy is a chronic disorder of the nervous system that causes individuals to have periodic or recurring seizures. These seizures are triggered by "sudden episodes of abnormal electrical activity in the brain" and can involve a blend of abnormal body functions, both physically and emotionally. While these seizures can be prompted by a specific type of stimulus, such as flashing lights, or can be traced back to a cause, such as a brain injury or a genetic trauma at birth, many cases of epilepsy do not have an obvious stimulus or cause; this type of epilepsy is known as ideopathic epilepsy.
Statistics from the Epilepsy Foundation
Common causes from Medline Plus, Medical Encyclopedia entry - Epilepsy
Triggers from Epilepsy Foundation, Triggers of Seizures