Most humans experience bouts of sadness from time-to-time. These periods are usually defined by a clear even or series of events, such as absence of a close friend or family member, loss, divorce, financial trouble, failing a test, or not getting a job. (This list is not inclusive.) When sadness is experienced, usually there is some relief associated with talking through the issue, venting or crying, and is usually able to be resolved in time.
Others experience pervasive sadness or depression, which has an overpowering effect on many parts of a person's life. In extreme cases, this can lead to harming oneself or others, suicide ideation, or worse.
Anyone who is experiencing extreme sadness or depression should seek proper medical attention. The resources here are designed to provide tools, strategies, and help teach coping skills for those who need less intensive support.