Basically Depression is low mood, or put another way, misery or unhappiness.
Sleep is often difficult, and appetite and sexual desire often drop off dramatically.
People may seem tearful, irritable and angry, lethargic, or even suicidal if they are seriously depressed. Sadly, about 1 in 6 severely depressed people kill themselves eventually. However, there are treatments that can help. There doesn't seem to be just one cause of depression, the list seems almost endless. As you can see, almost anything bad that has happened to you can cause depression.