Depression is a common mood disturbance characterized by feelings of sadness, disappointment, or loneliness that can lead to withdrawal from activities and loss of pleasure. It affects people in different ways and severities. Mild depression is brief and does not seriously interfere with daily life, while moderate depression causes hopelessness and interferes more, and severe depression involves detachment from reality. Treatment may include medication, counseling, and changes to one's environment, with the goal of easing symptoms and improving mood over time.