Anxiety is a normal emotion that everyone experiences from time to time. Many people feel anxious, or nervous, when faced with problems at work, before taking a test, or having to make an important decision. Anxiety is defined by Mirriam-Webster as "an abnormal and overwhelming sense of apprehension and fear often marked by physiological signs (as sweating, tension, and increased pulse), by doubt concerning the reality and nature of the threat, and by self-doubt about one's capacity to cope with it."
Anxiety disorders, however, are different. They can cause such distress that it interferes with a person’s ability to lead a normal life. Worry and fears are constant and overwhelming, and can be crippling. Some anxiety disorders are panic disorder, obsessive-compulsive disorder, post-traumatic stress disorder, social anxiety disorder, specific phobias, and generalized anxiety disorder. It is not known for certain what causes anxiety disorders, but it is guessed that it is a combination of environmental factors, genetics, medical conditions and psychological factors. Here is a very helpful article that discusses some examples of these factors that can play into the development of an anxiety disorder (read me). With proper treatment from a medical provider and counselor, the crippling caused by anxiety can be lifted and a person can feel like themselves again.