Agoraphobia is an anxiety disorder which primarily consists of the fear of experiencing a difficult or embarrassing situation from which the sufferer cannot escape.
People with agoraphobia may experience severe panic attacks in situations where they feel trapped, insecure, out of control, or too far from their personal comfort zone. In severe cases, an agoraphobic may be confined not only to their home, but to one or two rooms, and they may even become bed-bound, or a recluse.
Agoraphobia sufferers are often extremely sensitized to their own bodily sensations, subconsciously over-reacting to perfectly normal events. For example, the exertion involved in climbing a flight of stairs may trigger a full-blown panic attack, because it increases the heartbeat and breathing rate, which the agoraphobic interprets as the start of a panic attack instead of a normal fluctuation.

More info, help links & Source:
http://en.wikipedia.org/wiki/Agoraphobia