Seizure can also refer to the act of taking hold of property, for this meaning see Seizure (law)

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.  Some will, but there are also other kinds of seizures.

Types of Seizures 
There are many different types of seizures. They are named by how much of the brain they affect and what happens to the person when they are having that type of seizure.

Partial seizures 
In partial seizures, only a small part of the brain is involved in the seizure. These seizures can be more specifically called:
Simple partial: The person stays awake the whole time the seizure is happening.  They may twitch (especially in just one part of their body), feel dizzy, or smell and taste things that are not there.
Complex partial: The person "zones out" during the seizure and may seem confused, experience deja vu, laugh, feel afraid, see things that are not there, or smell something bad.  The person may also do something over and over again, like button and rebutton a shirt.

Generalized seizures 
In generalized seizures, a larger part of the brain is involved in the seizure.  Often, parts of both hemispheres (halves of the brain) are affected. These seizures can be more specifically called:
Tonic-clonic - The person faints, and starts having involuntary jerking motions. They may bite their tongue, scream, drool, urinate or defecate. Many people with this kind of seizures have a "strange feeling" before the seizure. This feeling is called aura. Tonic-clonic seizures can last up to 20 minutes.
Absence - People having absence seizures often look like they are just "spaced out."  They do not fall to the ground or have jerking movements, but they do not seem to hear or notice anything around them.  Other people may not notice that the person is having a seizure at all. The person may simply freeze in place and pick up where they left off when the seizure is over. The person having the seizure usually does not remember it afterward. This kind of seizure only lasts up to 10 seconds.
Myoclonic - A myoclonic jerk is a sudden jerking motion, usually on both sides of the body. This kind of seizure is most common in children under 5.   Myoclonic seizures can be seen in adults, who tend of have myoclonic jerks when they are falling asleep or already asleep.  Children can have these myoclonic jerks while awake.

Status epilepticus: A medical emergency 

Status epilepticus is a medical emergency.  A person is "in status" when:
 They have a seizure that lasts more than five minutes; OR
 They have more than one seizure without recovering in between.
Status epilepticus is a medical emergency because the brain will not get enough oxygen during a long seizure.  This can cause brain damage or death.

What Causes Seizures? 
Certain types of seizures point to a disorder called epilepsy, where the nerves do not work as they should. They carry the wrong messages to the brain so that the person moves uncontrollably or sees, hears, smells, feels, or tastes things that are not there. Medicine can be taken to prevent this from happening.

Other than epilepsy, many other things can cause seizures.

Illnesses
Diseases that can cause seizures include:
Infections in the brain, like meningitis (infection of the brain's lining) or encephalitis
Fever
This usually only causes seizures in children from 3 months to 6 years old.  Seizures that are caused by fever are called febrile seizures.  Children usually grow out of them.  However, an adult can have a seizure from a very high fever.
Brain tumor
Very high blood pressure
Stroke
Very low blood sugar levels (hypoglycemia)

Drugs and alcohol
Drugs and alcohol can cause seizures:
Certain types of medicines or drugs, or drug withdrawal (suddenly not taking a medicine or drug any more)
This includes suddenly not taking anti-seizure medicines (medicines that are supposed to prevent seizures)
Drinking too much alcohol
Overdosing on stimulant drugs like methamphetamine or cocaine
Alcohol withdrawal (when a person who normally drinks a lot suddenly stops drinking)
In the worst cases, this can cause a medical emergency called delirium tremens, which can cause status epilepticus

Other causes
Other things that can cause seizures include:
Injuries to the head or brain
Anything that causes the brain to not get enough oxygen, like drowning, being strangled, having a very bad asthma attack, having the heart stop, or smoke inhalation (breathing in too much smoke from a fire)
Heat stroke (a medical emergency that happens when the body and brain get very hot)

Possible first aid 
Most seizures only last a short time. The best thing to do is to prevent the person with the seizure from injury. During a seizure, reflexes do not work, and the people with the seizure do not have control over their muscles.

Related pages 
 Epilepsy, a condition that commonly causes seizures
 Status epilepticus
 Benign neonatal seizures (a cause of seizures in infants)
Aura - a strange feeling that occurs just before a seizure.

References

Neurology