I have an article for this too, but I am going to refer to an IRC buddy who have a much better article on this: http://www.gwern.net/Spaced%20repetition

In short, it basically exploits human cognitive architecture to provide an extremely efficient means of learning information. Two principles in operation are spaced repetition and the forgetting curve. Our knowledge tend to decay at an exponential rate. The trick is to study them right before you forget them. Thus, it free you from the drudgery of studying what you already know. It could be done by a human as a habit, but that isn't practical. That's why computers does it for us.

That's how Khan Academy works too, in regard to reviewing problems, or so I am told.