depression

http://www.utexas.edu/student/cmhc/booklets/depression/depress.html

Depression

Persistent sad, anxious, or "empty" mood
Feelings of hopelessness, pessimism
Feelings of guilt, worthlessness, helplessness
Loss of interest or pleasure in hobbies and activities that were once enjoyed, including sex
Decreased energy, fatigue, being "slowed down"
Difficulty concentrating, remembering, making decisions
Insomnia, early-morning awakening, or oversleeping
Appetite and/or weight loss or overeating and weight gain
Thoughts of death or suicide; suicide attempts
Restlessness, irritability
Persistent physical symptoms that do not respond to treatment, such as headaches, digestive disorders, and chronic pain

is persistant. it can be characterised by many different types of symptoms.

some people cry all the time for no reason, some people have a lack of any emotion and feel empty. it is the persistance over time that usually indicates whether or not you are suffering from it.

everyone has a different emotional range. but for those that get very down,
prevention is better than cure. if you feel you are spiralling downwards, or
are feeling down for no reason then take care of yourself. good nutritious food, exercise, eight hours sleep a night, good company, and avoid stress.
do things you like, and make sure you have a good support network of
friends and family that you enjoy being around and that make you feel good.

and remember that emotions are just that - emotions. they pass.
depression doesnt pass - it stays and stays until you get treatment.

not to make light of your feelings, take care of yourself. if you are suffering from depression following the recommendations in that link will really help, and even if you are not, they are still good to follow!!!