Everyone occasionally feels blue or sad. But these feelings are usually short-lived and pass within a couple of days. When you have depression, it interferes with daily life and causes pain for both you and those who care about you. Depression is a common but serious illness.

Many people with a depressive illness never seek treatment. But the majority, even those with the most severe depression, can get better with treatment. Medications, psychotherapies, and other methods can effectively treat people with depression.
There are several forms of depressive disorders.

Major depression where the severe symptoms interfere with your ability to work, sleep, study eat and enjoy life. An episode can occur once in a person’s lifetime, but more often, a person has several episodes.

Persistent depressive disorder where the depressed mood lasts for at least 2 years. A person diagnosed with persistent depressive disorder may have episodes of major depression along with periods of less severe symptoms, but symptoms do last for 2 years.
Some forms of depression are slightly different, or they may develop under unique circumstances. They include:

• Psychotic depression, which occurs when a person has severe depression plus some form of psychosis, such as having disturbing false beliefs or a break with reality (delusions), or hearing or seeing upsetting things that others cannot hear or see (hallucinations)
• Postpartum depression, which is much more serious than the “baby blues” that many women experience after giving birth, when hormonal and physical changes and the new responsibility of caring for a newborn can be overwhelming.
• Seasonal affective disorder (SAD), which is characterized by the onset of depression during the winter months, when there is less natural sunlight. The depression generally lifts during spring and summer

Most likely, depression is caused by a combination of genetic, biological, environmental, and psychological factors.

Signs and symptoms include:

• Persistent sad, anxious, or “empty” feelings
• Feelings of hopelessness or pessimism
• Feelings of guilt, worthlessness, or helplessness
• Irritability, restlessness
• Loss of interest in activities or hobbies once pleasurable, including sex
• Fatigue and decreased energy
• Difficulty concentrating, remembering details, and making decisions
• Insomnia, early-morning wakefulness, or excessive sleeping
• Overeating, or appetite loss
• Thoughts of suicide, suicide attempts
• Aches or pains, headaches, cramps, or digestive problems that do not ease even with treatment.

Depression, even the most severe cases, can be effectively treated. The earlier that treatment can begin, the more effective it is.

Once diagnosed, a person with depression can be treated in several ways. The most common treatments are medication and psychotherapy.

Excerpt from http://www.cadabamshospitals.com/patient-care/depression.html