Most people deal with bouts of feeling depressed at some point in their lives, but this doesn't mean that they have clinical depression. If you're exhibiting symptoms of depression (lack of motivation, decreased energy, as well as feelings of hopelessness and of guilt) for longer than 2 weeks, you may be dealing with clinical depression.
Many outsiders look at someone who's clinically depressed and say, "Just pull yourself together and get over it." But, what they don't understand is that it's not that simple. Yes, it's a psychological condition, but because it's deeply biological and because genetics play a role, it's similar to telling someone who's stricken with a physical impairment of some kind, to cure themselves of it.