Everybody feels down from time to time, and that's normal. When you can't function at all for weeks, months or years on end, then it's clearly a disorder. "Can't function" can mean an inability to: get out of bed, to sleep normally, to bathe or otherwise take care of yourself, to experience pleasure, to eat normally, to work, and/or to concentrate. Usually these are combined with overwhelming feelings of: despair, worthlessness, hopelessness and/or isolation. Sometimes these people have suicidal thoughts and act on them.

People who have never experienced a mental illness often find it hard to understand what they are.