Epilepsy is a brain disorder that causes siezures due to the sudden discharges of energy by the nerve cells of the brain. Seizures are episodes of disturbed brain activity that cause changes in attention or behavior. There are two types of epileptic siezures, generalized and partial. A generalized seizure effects the whole brain while a partial only effects part of the brain.
Generalized seizures are identified by absence, commonly known as "blanking out", grand mal, the loss of conciousness and convulsions, and sudden jerking movements.
Partial seizures cause twitching of a leg or arm, vision distortions, hearing distortions, and altered emotions, but the person remains concious.
Some people also experience a feeling commonly known as an aura. To explain more, a quote from an article on epilepsy:
"Some people experience an aura, an unusual feeling in the stomach, a sense of fear, or other sensory change, before an observable seizure occurs. The aura is the beginning of a simple partial seizure, which then spreads to involve a larger area of the brain."
Scherer, Ann. "Epilepsy." Grolier Multimedia Encyclopedia. Grolier Online, 2011. Web. 5 Dec. 2011.