Skip to about 1:40 if you like, it's all intro up until then.
I bumped into this over on Hacker News, where various people were saying that this was much clearer than any of the explanations they got about _anything_ in school. It's a shame that none of the material I've seen for online training has been this clear. (Although it's been a few years since I looked at one, quality may have vastly improved.)
Original post on Dreamwidth - there are comments there.