ok, from the definition, and drawing a picture, I can understand what all these mean. And when it comes to solving problems, I can solve them too. i.e 2x/1-x = 2y/y-1. easy, I know how to do it. But when it comes ot applying it to finite/infinite sets, I dont know how to start. i.e, is there a bijection from the positive real number to the set of natural numbers. ok, so positive real numbers imply 0, 1, 2,....n, and also everything in between. natural number imply 1, 2, ......n. so what does this mean? what does it mean if there is a bijection between these two? injection? surjection? how do I tink this through?