Major Depressive Disorder is a mood disorder characterized by at least two weeks of depressed mood, feelings of worthlessness, and diminished interest in activities. Signs include lethargy, feelings of worthlessness, and loss of interest in friends and activities. Cognitive symptoms include low self-esteem, pessimism, and exaggerated negative thoughts. Treatment involves cognitive therapy to challenge negative thought patterns and develop supportive relationships, regular exercise, a healthy diet, and medication if needed.