The seizures in epilepsy may be related to a brain injury or a family tendency, but most of the time the cause is unknown. The word "epilepsy" does not indicate anything about the cause of the person's seizures, what type they are, or how severe they are. Almost any type of behavior that happens repetitively may represent a seizure, but they are usually defined in three categories:

Generalized seizures: All areas of the brain (the cortex) are involved in a generalized seizure. Sometimes these are referred to as grand mal seizures.

• To the observer, the person experiencing such a seizure may cry out or make some sound, stiffen for some seconds, then have rhythmic movements of the arms and legs. Often the rhythmic movements slow before stopping.
• Eyes are generally open.
• The person may not appear to be breathing. The person is often breathing deeply after an episode.
• The return to consciousness is gradual and should occur within a few moments.
• Loss of urine is common.
• Often people will be confused briefly after a generalized seizure.

Partial or focal seizures: Only part of the brain is involved, so only part of the body is affected. Depending on the part of the brain having abnormal electrical activity, symptoms may vary.

• If the part of the brain controlling movement of the hand is involved, for example, then perhaps only the hand may show rhythmic movements or jerking.
• If other areas of the brain are involved, symptoms might include strange sensations or small repetitive movements such as picking at clothes or lip smacking.
• Sometimes the person with a partial seizure appears dazed or confused. This may represent a partial complex seizure. The term complex is used by doctors to describe a person who is between being fully alert and unconscious.

Absence or petit mal seizures: These are most common in childhood.

• Impairment of consciousness is present with the person often staring blankly.
• Repetitive blinking or other small movements may be present.
• Typically, these seizures are brief, lasting only seconds. Some people may have many of these in a day. Other seizure types exist particularly in very small children.

The reasons why epilepsy begins are different for people of different ages, but the cause is unknown for about half of all cases. Children may be born with a defect in the structure of their brain, or they may suffer a head injury or infection that causes their epilepsy. Severe head injury is the most common known cause in young adults. In middle age, strokes, tumors, and injuries are more frequent. In people over 65, stroke is the most common known cause, followed by degenerative conditions such as Alzheimer's disease.
Often seizures do not begin immediately after a person has an injury to the brain. Instead, a seizure may happen many months later. We do not have a good explanation for this common observation, but scientists are actively researching this subject.