Symptoms:

persistent sad, anxious, or "empty" mood
feelings of hopelessness, pessimism
feelings of guilt, worthlessness, helplessness
loss of interest or pleasure in hobbies and activities that you once enjoyed, including sex
insomnia, early-morning awakening, or oversleeping
appetite and or weight loss or overeating and weight gain
decreased energy, fatigue, being "slowed down"
thoughts of death or suicide, suicide attempts
restlessness, irritability
difficulty concentrating, remembering, making decisions
persistent physical symptoms that do not respond to treatment, such as headaches, digestive disorders, and chronic pain

Causes:

genetic factors: if there is a family history of depression, but not always
biochemistry: chemical imbalance of serotonin, norepinephrine and cortisol; some medications such as reserpine
environmental stresses: significant loss, difficult relationship, financial problems, major life change in life pattern, acute or chronic physical illness, and substance abuse disorder
psychosocial factors: pessimistic thinking, low self-esteem, a sense of little control over life events, excessive worrying.

Treatment:

An evaluation including a medical/psychiatric history is done before any treatment is administered.

Medications: heterocyclic, serotonin reuptake blockers, MAO inhibitors, and lithium.
Psychotherapy: verbal interaction between a trained professional and a patient with emotional or behavioral problems.
Interpersonal Psychotherapy: therapist helps the patient understand his or her illness and how depression and interpersonal conflicts are related
Cognitive/Behavioral Therapy: therapist uses talk therapy and behavioral prescriptions to alleviate negative thought patterns and beliefs.
Psychoanalysis: therapist meets with the patient to identify and resolve the patient's past conflicts that have given rise to depression in later years.
Psychodynamic Psychotherapy: similar to psychoanalysis, that recognizes the significant effects that emotions and unconscious motivation can have on human behavior.
Electroconvulsive Therapy: patient receives anesthesia and a muscle relaxant, then electrodes are placed on the head and a small amount of electricity is applied. This procedure is only used when medication does not work or can not be taken and is repeated 2-3 times a week until the patient improves or the treatment becomes ineffective.

Self - Help

realize negative views will go away as treatment takes effect
participate in support groups
mild exercise
set small goals
work on small task
do not expect too much from yourself
try to be with people
do not make major life decisions
participate in activities that will make you feel better
do not expect to "snap out" of your depression
do not accept your negative thinking