Anxiety is a feeling of apprehension and fear characterized by physical symptoms such as

• palpitations,
• sweating,
• irritability,
• feelings of stress.

Anxiety disorders are serious medical illnesses that affect approximately 19 million American adults. In fact, anxiety disorders as a group are the most common mental illness in America. Anxiety disorders can affect adults, children, and adolescents.

These disorders fill people's lives with overwhelming anxiety and fear. Unlike the relatively mild, brief anxiety caused by a stressful event such as a business presentation or a first date, anxiety disorders are chronic, relentless, and can grow progressively worse if not treated. People who suffer from anxiety disorders typically struggle with difficult symptoms such as agitation, feeling "uptight," worry, and apprehension on a daily basis. These disturbing symptoms can become so severe that they interfere with normal daily activities. Sometimes anxiety disorders lead to restlessness, poor sleep and insomnia, trouble concentrating, feeling tense, a sense of dread, chest pain, lightheadedness, trouble breathing, hyperventilation, and even overwhelming panic with a feeling of losing control.

Source: http://www.rxlist.com