A convulsion is a medical condition where body muscles contract and relax rapidly and repeatedly, resulting in an uncontrolled shaking of the body. Because a convulsion is often a symptom of an epileptic seizure, the term convulsion is sometimes used as a synonym for seizure. However, not all epileptic seizures lead to convulsions, and not all convulsions are caused by epileptic seizures. Convulsions are also consistent with an electric shock and improper Enriched Air Scuba Diving. For non-epileptic convulsions, see non-epileptic seizures.
The word "fit" is sometimes used to mean a convulsion or epileptic seizure.
When a person is having a convulsion, they may experience several different symptoms. These may include: a brief blackout, confusion, drooling, loss of bowel/bladder control, sudden shaking of entire body, uncontrollable muscle spasms, temporary cessation of breathing, and many more. Symptoms usually last from a few seconds to around 15 minutes. If someone has a fit like this, you should help to make sure they don't fall and injure themselves, cushion their head and loosen any restricting clothing/jewelry, and also call for medical help. 
Convulsions are often caused by some sort of electrical activity mishap in the brain. Often times, the cause is not able to be pinpointed. Convulsions can be caused by chemicals in the blood, as well as infections like meningitis or encephalitis. A very common cause of convulsions is fevers. Other possibilities include head trauma, stroke or lack of oxygen to the brain. Sometimes the convulsion can be caused by genetic defects or brain tumors. 
The most common type of seizure is called a Grand Mal Seizure, also known as a generalized convulsion. This is characterized by a loss of consciousness which may lead to the person collapsing. The body stiffens for about a minute and then jerks uncontrollably for the next minute. During this, the patient may fall and injure themselves or bite their tongue and lose control of their bladder. A familial history of this puts a person at a greater risk for developing them.
A common question about convulsions is whether or not they cause harm to the brain in the long run. It has been found that they can have a negative impact on cognition. Some research studies have found that subjects who have frequent convulsions have had decreased cognitive abilities as well as reduced verbal and visual-spatial abilities. The degree of complications arising from the convulsions depends on the location in the brain that is affected. The longer a patient suffers from convulsions, the more damage they are likely to have accumulated.