I'm not a doctor or a psychologist, but here's an attempt to explain it in a nutshell: Depression comes in many forms and degrees of severity, but common symptoms include long periods of time with inability to enjoy the things you usually enjoy, inability to get out of bed in the morning, constant crying, constant fatigue and malaise, sleeping too much or too little over long periods of time, etc. One or two days of being down does not usually count as depression, so far as I know, unless the patient is feeling self-destructive and/or suicidal. There are many physical causes of depression, so usually seeing a doctor first is a good idea in order to rule out medical causes of depression, such as failure of the adrenal glans or insufficient levels of seratonin. Getting into counseling is also recommended, in tandem with having a complete physical exam with blood work to see if your blood chemical levels are normal. Hope this was helpful.
"Thou, O Lord, are the shield about me;
You're my helper,
You're the one who lifts up my head."
"Whom have I in heaven or earth but You?"