It’s such a cool name… I had to look it up.
Apparently since the early days of IE, there were are sorts of issues with attaching arbitrary attributes (of your own) to DOM elements. Well, not unless you use…Expando….ACTIVATE! Zap! Expando properties!
So anyway, I discovered this because jQuery mentioned it. Seems that even with expandos, you still can’t really attache whatever you want. So the folks over there gave us this wonderful “data” thing, so you don’t have to worry much about dear old, old, dysfunctional IE.