Fainting can be viewed as a less severe form of unconsciousness, although the causes of these 2 conditions are different.

Fainting

Fainting is a brief loss of consciousness. It occurs when the blood supply to the brain is insufficient. The reasons for this reduced supply of blood could be due to overexertion, emotional shock or lack of movement (think about the people who collapse when standing still during a foot drill parade). You might have experiences a lack of blood supply to your brain before. It could have happened after you have been sitting down for a long time and suddenly stand up. You would feel dazed for a couple of seconds and require a conscious effort to maintain your balance. If you had such an experience, it is because of the lack of blood in your brain.

Symptoms include cold sweat, dizziness and nausea. The victim may also be semi-conscious, being unable to respond to verbal instructions. A drop in pulse or blood pressure can also be found. Sometimes before the person falls to the ground, you will see him swaying or unable to balance himself.

Fainting could last for a period of a couple of seconds to several minutes. Recovery is achieved with an increase in blood flow to the brain.

Unconsciousness

Unconsciousness is a more severe form of loss of consciousness. An unconscious person will not be aware of his surroundings. An unconscious person will be lying on the floor or sitting in a chair.  The causes of unconsciousness includes a severe blow to the head, lack of oxygen to the brain or poisoning. For example, a person stuck in a stationary car should suffer from carbon monoxide poisoning and fall unconscious.

Coma is a form of unconsciousness, although it is considered to be a long-period of being unconscious.