What is the difference between being asleep and being unconscious?

Unconsciousness is the condition of being not conscious—in a mental state that involves complete or near-complete lack of responsiveness to people and other environmental stimuli. Being in a comatose state or coma is a type of unconsciousness. Fainting due to a drop in blood pressure and a decrease of the oxygen supply to the brain is a temporary loss of consciousness. Loss of consciousness must not be confused with altered states of consciousness, such as delirium (when the person is confused and only partially responsive to the environment), normal sleep, hypnosis, and other altered states in which the person responds to stimuli.

https://en.wikipedia.org/wiki/Unconsciousness

Unconsciousness is when a person is unable to respond to people and activities. Often, this is called a coma or being in a comatose state.

Other changes in awareness can occur without becoming unconscious. Medically, these are called "altered mental status" or "changed mental status." They include sudden confusion, disorientation, or stupor.

Unconsciousness or any other SUDDEN change in mental status must be treated as a medical emergency.

If someone is awake but less alert than usual, ask a few simple questions, such as:

What is your name?
What is the date?
How old are you?

Wrong answers or an inability to answer the question suggest a change in mental status.

Being asleep is not the same thing as being unconscious. A sleeping person will respond to loud noises or gentle shaking -- an unconscious person will not.

An unconscious person cannot cough or clear his or her throat. This can lead to death if the airway becomes blocked.

http://www.nlm.nih.gov/medlineplus/ency/article/000022.htm