Depression is a serious disease. It is not anyone's fault, and if you are depressed, you are not weak, lazy, stupid, undisciplined, forgetful, or any of the other things that voice in your head is saying you are. You’re just someone who is having a hard time dealing with things at the moment. The good news is that you can and will recover.
Sadness is a normal part of our lives. Often, we feel sad because of stressful things that happen to us (the death of a loved one, the breakup of a relationship, serious illness, the loss of a job). Depression is sadness that is much more intense than, or lasts much longer than, the ordinary sadness we might expect to feel under the specific circumstances. Also, while we may feel depressed as a reaction to a specific, stressful event in our lives, we can also be depressed even though no "major" stressful event has happened.
One key difference between depression and ordinary sadness is that depression often impairs our ability to function. Depression results from chemical action in the brain, which is why anti-depressant medications can help treat it.
Someone who suffers from depression is said to be "clinically depressed," or to have a "unipolar disorder." Note that a depressed person may not be depressed all the time, but may simply be depressed at certain times. Some people even have depression that alternates with extreme "up" moods: they are said to have a "bipolar disorder," or be "manic-depressive." Even people with a bi-polar disorder may be depressed enough to be suicidal when they are having a depressive episode.
Depression may have a number of causes including:
...the effects of some medications
...alcohol or drug abuse
Because of this, it is usually a good idea for someone who suffers from depression to see a doctor to make sure that the depression does not have a medical cause.