To be honest, back when I was in high school all I understood it to be was the workload. That is, amount of work you had to do in your class went Regular < AP < IB. Many of the IB classes at my HS were actually both AP/IB courses, save for a select few, like the Theory of Knowledge course.

I was "only" an AP student, but I will say that based on the friends I had in IB and the people I saw coming out of those classes, IB students generally tended to be very intelligent in the academic sense of the word, more so than the majority of the AP students. That's the extent of my knowledge (opinion?).