Many people use the word "depression" commonly and almost loosely, but depression is a clinical term that involves different symptoms based on its various forms. Depression is different from being sad or blue; it does not simply pass within a couple of days and requires treatment.
Here are the following types of depressive disorders, as listed by the National Institute of Mental Health (NIMH).
Major depression,-severe symptoms that interfere with your ability to work, sleep, study, eat, and enjoy life. An episode can occur only once in a person's lifetime, but more often, a person has several episodes.
Persistent depressive disorder-depressed mood that 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 must 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. It is estimated that 10 to 15 percent of women experience postpartum depression after giving birth.
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. SAD may be effectively treated with light therapy, but nearly half of those with SAD do not get better with light therapy alone. Antidepressant medication and psychotherapy can reduce SAD symptoms, either alone or in combination with light therapy.
From NIMH.NIH.gov