What is the difference between natural variation and natural selection?

Variation is simply the differences between individuals of a species. E.g. we are all humans but there are variations which make us all different. Natural selection is the process by which the individuals best adapted to survive, breed and pass on their genes. Those not suited to an environment will eventually die out. Some variations may be advantageous therefore there may be more of those organisms around and therefore those will be most prevalent in that habitat. Some variations may be detrimental and cause those numbers to decline.