Explains how i3 works internally so that you can fully understand what's going on. I expect you to have watched the screencast on v4.1 already.
This video (1 hour) is an introduction to i3, explaining our motivation, a bit of history, some cool features and how we develop.
This three-part series of screencasts created by Alex Booker illustrates what i3 is, how to use it, how to configure it, and how to customize its appearance. (1 hour, 40 minutes)
To get a quick impression of i3, have a look at these screenshots.