Meaning of area under ROC curve?

I know that area under ROC curve 0.8 is better than 0.7. But in words, there is ___% chance to rate True positives in 0.8 than in 0.7 form false +ves.Meaning of area under ROC curve?
http://en.wikipedia.org/wiki/Receiver_op鈥?/a>





says the area is ';is equal to the probability that a classifier will rank a randomly chosen positive instance higher than a randomly chosen negative one.'; No idea why this is.