Mood disorders are about feeling way down or way up to the extent that your life is miserable to you. Your emotional state is not stable.
Depression includes some or all of the following symptoms;
anhedonia (loss of pleasure)
feelings of hopelessness
lack of initiative
loss of appetite
Depression comes from external events like losing a job or feeling empty inside because you're not quite aligned with your true nature and true purpose in life.
The mental health experts say it could be caused by a neurochemical imbalance in your brain but you create your biochemistry through how you think and live. You can create happiness through what you think and do.
Your moods could change seasonally. This is called seasonal affective disorder – SAD.
A bipolar disorder is when your depression gives way to an excessive feeling of joy and energy. Such mood swings might happen very quickly.