Depression is something that affects all of us at some point and understanding what's going on in the body is useful to explain depression as a whole. The folks at AsapSCIENCE break it down.
While the science here is interesting and concentrates on how we think depression is linked heavily to genes, the important takeaway is that depression is a disease with a biological basis. This is to say it's not just something people can "get over". If you, or someone you know is depressed, understanding what's really going on in the body is an important part of dealing with it.
The Science of Depression [YouTube]
If depression is affecting you or someone you know, call Lifeline on 13 11 14.