The worst emotion you could ever feel. Every day becomes a living hell and you wonder if you have a purpose on this earth and why you should even be alive anyway. Depression will cause you to be on the edge of tears every minute of the day, and you eventually just stop caring about everything. Relationships suffer, friendships end, and suddenly you feel alone and there's no one but depression to keep you company. Sometimes depression is mistaken for melodrama, and the person who you think is crying for attention is actually screaming on the inside. Depressed people wish they could end the agony, but can't because there aren't many alternatives.
Depression usually doesn't have just one cause. It's a whole load of little problems that pile up after an unspecific length of time(depending on the problems, and how long you've kept your feelings bottled up inside) and eventually grow to become unbearable. You feel trapped and you want out but there's no way out, and every single simple everyday task like waking up and going to school becomes futile in your mind.
Just as I was reading these definitions for the same word, there were tears rolling down my face. Looks like I've got depression again. This is pathetic.