Very high level, but I think <a href="http://www.r2d3.us/" rel="nofollow">http://www.r2d3.us/</a> is actually a pretty good visual intro, although it skips over information gain/CART metrics.
I'm usually not for people pushing books on HN like this, but this is entirely too relevant to my interests not to thank you for posting this. Visualizing my processes has by far been the hardest part of getting my CS degree, and I have a professor this semester as well as next that is obsessed with machine learning.