Depression is a disorder that is evidenced by excessive sadness, loss of interest in enjoyable things, and low motivation.
It is normal to experience feelings of sadness and dispair in response to adverse life events. Such events could include loss, major life changes, stress, or disappointment. In most cases, the sad feelings resolve as you come to terms with the changes in your life. In situations such as bereavement, these feelings may persist for months and return at significant times, such as birthdays and anniversaries related to the lost loved one. Provided you have times when you can enjoy things, however, this sadness is not a sign of depression.
Depression is common. One in three people will experience a major depressive episode at some stage in their lives. While most cases of depression are mild, about one person in ten will have a moderate or severe episode.