I didn’t even know there was a different term for it. It just means “being in a bad mood” and is a form of mild depression (ie just a step below clinical depression).
It is the same type of illness and has the exact same symptoms as depression. I guess the only real difference is that if you constantly find yourself in a bad mood, you (or your friends) could help the situation by changing lifestyle, starting a hobby or just eating chocolate, whereas depression must be treated by a professional.