Just what is XLR and how does it relate to balanced audio?

I've been using XLR cables for quite a few years, but recently I felt compelled to delve into what XLR is, and why it is used so universally in professional audio and video. And what is balanced audio? are the cables balanced? or is it the electronics? How does it work to eliminated noise in cables, and how does the noise get there in the first place? I hope to have answered some of these questions in this video.