Anxiety is a normal reaction to stress. It helps one deal with a tense situation in the office, study harder for an exam, keep focused on an important speech. In general, it helps one cope. But when anxiety becomes an excessive, irrational dread of everyday situations, it has become a disabling disorder.
Five major types of anxiety disorders are:
(http://www.nimh.nih.gov/health/topics/anxiety-disorders/index.shtml)