Vestigial Structures

A vestigial structure is simply a structure that is either useless to the organism, or not used for it's intended purpose. For example, penguins have wings although they are flightless. Yes, penguin wings are useful for swimming (except for when it comes to the lazy penguins at the zoo) but a penguin could survive without it's wings. It's important for vestigial structures to be discovered because they often show clues as to the ancestry of an organism.
[PS: That strange thing at the top of the picture is Harry Potter's head. I couldn't get his whole face in the shot. Whoops.]