The National Institute of Mental Health has a great article on depression's definition, causes, symptoms, treatments, and more. To sum it up, depression is when you feel so sad that you can't function normally anymore. Nobody knows exactly what causes depression, but there seem to be a lot of contributing factors. Also, there are a lot of things you can do to treat depression.
My goal is to look in-depth at each of the potential contributing factors and treatments of depression, so stay tuned for more.