The "real metal" is what you are seeing. It has just been pitted due to the acid bath and the surface has been slightly altered.
To give you the Readers Digest condensed version this is kinda sorta what happens: Nickels are an alloy of mostly copper and nickel. When the coin is struck with the dies, the densities of those metals are different in detailed and non detailed areas of the struck coin. When the coin is subjected to the acid, it attacks the copper and nickel at different rates. As the metals are removed at different rates the details of the original coin reappear like magic.
Yes, people do collect them.