Most difinitely NOT a mood; it is NOT something that one can get over by friends making them laugh. Depression is an illness just like diabetes or cancer that affects the mind and needs to be treated just as urgently. Can be fatal. It is a horribly unbearable feeling inside that does not go away.
Take a deep breath in, hold it, take another breath in. That pain that you feel is the start of depressive feelings. Now pretend you haven't slept in 4 weeks and your body hurts like you have a flu. That is sorta like what depression feels like