Depression is an illness that involves the body, mood, and thoughts and affects the way a person eats and sleeps, the way one feels about oneself, and the way one thinks about things. Depressive disorders are characterized by pervasive mood changes that affect all aspects of an individual's daily functioning. A depressive disorder is not the same as a passing blue mood and is more than a case of persistent sadness.

Symptoms of depression also include

• feelings of worthlessness,
• hopelessness,
• helplessness,
• guilt,
• lack of interest in daily activities,
• irritability,
• loss of energy,
• loss of appetite,
• sleep problems,
• self-loathing,
• thoughts of suicide.

Depression can happen for unknown reasons. Depression can also be caused by medications, pregnancy, postpartum state, abuse, associated illness, chronic stress, seasonal affective disorder, and other conditions.

Symptoms of depression also include feelings of worthlessness, hopelessness, guilt, lack of interest in daily activities, and even thoughts of suicide. Depression is not a sign of personal weakness or a condition that can be wished away. People with a depressive disease cannot merely "pull themselves together" and get better. Without treatment, symptoms can last for weeks, months, or years. Appropriate treatment, however, can help most people with depression.

Source: http://www.rxlist.com