Healthy levels of anxiety help to motivate us to do things. There is a point beyond which the anxiety experienced by a person is detrimental to their health and well being rather than a positive influence.
A phobia is an intense anxiety about a particular event or thing that is so overwhelming for someone that this person is not able to function as they usually can when they experience this phobia.
Anxiety is experienced by people in many ways. There are somatic or body symptoms such as shallow fast breathing, tingling in fingers, butterflies in the stomach, nausea,dry retching an an intense feeling of fear. Sometimes the anxiety builds up to an extreme intense negative overwhelming feeling called a panic attack.
During a panic attack, a person often believes they are going to die and there is a desperate need to remove themselves from the precipitating circumstances of the panic attack. A panic attack is such a terrifying experience that many people are unable to face the situation that the panic attack first occurred. Some people develop avoidance behaviours and sometimes struggle to go out from their safe environments. If people are unable to leave their home this is called agoraphobia.
If the anxiety is very specific to particular events or situations , then a graded exposure program can be helpful to work towards overcoming this anxiety.
Other psychological treatments and psychotherapy as well as medication can also help.