You are asking an excellent question. There are, in fact, two overall categories of depression: situational depression and biologically-based depression. A person may experience one or both of these and sometimes a combination of these two types. This classification system refers to the origin or basis of the depressive illness. The symptoms may be the same, but there are important differences between situational and biologically-based forms of depressive illness.
Just as the name implies, situational depression is triggered by a difficult life situation. It may be any type of situation that results in profound discouragement, sadness or loss. Examples includes a variety of experiences, such as being a victim of sexual assault, long-term emotional or physical abuse, a car accident, death of a loved one, relationship loss, long-term unemployment, personal illness or injury, chronic pain or any number of other painful life events. The initial reaction to such situations are usually grief, anger or a combination of these. Grief and anger are not the same as depression, but prolonged grief or a chronically stressful situation over time can turn grief into depression.
Biological depression often results from an inherited genetic predisposition toward depression (or anxiety), but can also be caused by traumatic brain injury and possibly related to the chronic abuse of hard drugs (such as methamphetamines) and alcohol. Biological depression can range from mild to severe. Situational depression is usually connected to a triggering event, while biological depression may involve a trigger or may occur entirely independently of any trigger. Most depressive illnesses develop gradually, though they may also occur rather suddenly in some circumstances.
Both types of depression are best treated with therapy and sometimes, but not always, require the use of an anti-depressant medication. Anti-depressant medications are most frequently needed in cases of biological depression or situational depression that has become more entrenched and biological in nature. Situational depression sometimes activates underlying biological depression.
When a person has biologically-based depression or has experienced several depressive episodes in their life, long-term medication treatment may be needed, but most situational depression does not require long-term use of anti-depressant medication. The good news is that both types of depression are highly treatable, so if you or someone you know is suffering from any form of depression it is wise to seek the advice of your general physician or a mental health professional.