Our brain runs on electricity. Usually, this electricity is well controlled, and runs smoothly from one part of the brain to another, just like it would in a big city like Mumbai. Sometimes there is a loss of control over this electricity. You can think of it as a short-circuit and small sparks flying in the brain.
When there is a short-circuit in one part of Mumbai, often the electricity in that area goes down. The electrical disturbance can keep spreading and sometimes the electric supply in the entire city is disrupted.
A seizure is caused by uncontrolled electrical activity inside the brain.
If the uncontrolled sparking occurs in the part of your brain that controls smell, then you may start smelling strange things. If it occurs in the part of your brain that moves the hand, then you can get jerking of the hand. If the seizure spreads all over your brain, then the entire body can start shaking vigorously. At this stage, the vigorous shaking of the body is called a “convulsion”.
Therefore, a convulsion indicates that a huge seizure is happening in the brain. Smaller seizures produce milder symptoms.