Depression is a common but serious illness that affects the body, mood, and thoughts. Depressive disorders are not the same as passing feelings of sadness, the blues or a sign of personal weakness. People with a depressive disorder cannot simply pull themselves together and get better. Yet, many people do not realize that depression is a highly treatable illness. In fact the majority of people suffering from depression, even those with the most severe forms of the illness, can get better if properly treated.

Depression is the most common mental health disorder after anxiety disorders. It affects 7% of adults in the United States in any given year, with a lifetime prevalence of 21% of all Americans. According to the World Health Organization, it is the leading cause of worldwide disability. Men and women may experience depression differently. Women are affected at twice the rate of men, while men with depression are more likely to die by suicide.

There are several forms of depressive disorders. The most common types include:

• Major Clinical Depression, which interferes with a person’s ability to work, sleep, study, eat, and enjoy once-pleasurable activities. Sometimes people with a major clinical depression can lose touch with reality and experience psychosis. This is called a psychotic depression, which can involve hallucinations (seeing or hearing things that aren't there) or delusions (false beliefs that aren't shared by others), such as believing they are bad or evil, or that they're being watched or followed. Individuals can also experience paranoia, feeling as though everyone is against them or that they are the cause of illness or bad events occurring around them.

• Dysthymia, which is a less severe form of depression but may last longer and may keep a person from functioning well or feeling good

• Bipolar Disorder, also called manic-depressive illness, which is characterized by cycling mood changes—severe highs (mania) to severe lows (depression).

• Cyclothymic Disorder, often described as a milder form of bipolar disorder. An individual can experience chronic fluctuating moods over at least two years, involving periods of hypomania (a mild to moderate level of mania) and periods of depressive symptoms. A person with cyclothymic disorder will experience only very short periods (no more than two months) of normality between the periods of hypomania and depression. The duration of the symptoms are shorter, less severe and not as regular, and therefore don't fit the criteria of bipolar disorder or major depression.

• Seasonal affective disorder (SAD) , a mood disorder that has a seasonal pattern. The cause of the disorder is unclear, but it's thought to be related to the variation in light exposure in different seasons. It's characterized by mood disturbances (either periods of depression or mania) that begin and end in a particular season. Depression which starts in winter and subsides when the season ends is the most common. It is usually diagnosed after the person has had the same symptoms during winter for a couple of years.

• Post Partum Depression (PPD), can be experienced in three forms: post partum blues, (baby blues); post partum depression; and postpartum psychosis, a highly dangerous form of PPD for both mother and baby which can result in hospitalization. Post Partum Depression is complex and is often the result of a combination of factors. In the days immediately following birth, many women experience the 'baby blues' which is a common condition related to hormonal changes and affects up to 80 per cent of women. The 'baby blues', or general stress adjusting to pregnancy and/or a new baby, are common experiences, but are different from depression. Post partum depression is longer lasting and can affect not only the mother, but her relationship with her baby, the child's development, the mother's relationship with her partner and with other members of the family.

Depression differs from sadness or grief/bereavement.
The death of a loved one, loss of a job or the ending of a relationship are difficult experiences for a person to endure. It is normal for feelings of sadness or grief to develop in response to such situations. Those experiencing loss often might describe themselves as being “depressed.” But being sad is not the same as having depression. The grieving process is natural and unique to each individual and shares some of the same features of depression. Both grief and depression may involve intense sadness and withdrawal from usual activities. Yet, grief and depression are different. In grief, sad and painful feelings will often mix with positive memories. In major depression, mood and interest in pleasure are significantly decreased for most of 2 weeks.

In addition, when one is experiencing grief or feelings of bereavement, self-esteem is usually maintained. In major depression, feelings of worthlessness and self-loathing are common. Distinguishing between depression and grief/bereavement is important because it can help people get the proper help, support or treatment they need.