A seizure happens when the nerves in a person's brain act strangely. Nerves send information, partly through electrical signals. Usually, nerves in the brain (called neurons) do not fire at the same time. During a seizure, groups of nerves start firing together, too fast. This causes there to be too much disorganized electrical activity in the brain.
Most people think a person with a seizure will shake and twitch.
Seizure Facts for Kids. Kiddle Encyclopedia.