Phobia is an irrational or obsessive fear of certain situations or objects. If a person is suffering from high level of anxiety, the person is at the verge of developing phobia for that thing. Claustrophobia is an anxiety disorder which usually results to an uncontrollable and irrational fear of tight or enclosed spaces.
You might have experienced or seen many people who would start developing a great level of anxiety when they find themselves in a very tight place or enclosed place like a small room, aeroplane, cave. Many of them would prefer to use stairs in a building that has elevator, many will even avoid getting stuck with people in a crowded place.
You will see some trying to stay very close to the door always, a claustrophobic person will prefer not go out in a car once there is traffic. A claustrophobic person might be showing the following symptoms; sweating, shaking, nausea, overbreathing, fainting etc.