Skeleton - Unlike the bony fishes of the world, sharks have a unique skeleton mainly composed of cartilage. Sharks do have some bones, but it is limited to their spinal column. Cartilage is a flexible yet sturdy material that allows the shark to maintain it's shape but to remain light and maneuverable. Cartilage can be found in the human body as well, just wiggle your ears or the tip of your nose. Gills - The gills of sharks are also different then bony fishes.
Bony fishes have a operculum or gill cover that pumps water across their gills.
Sharks do not have an operculum. But this doesn't mean sharks have to constantly be in motion in order to survive. Some sharks have learned to lie in currents that flow past their gill slits. Skin - A sharks skin is made out of placoid scales. They are tooth like structures which are abrasive if rubbed from back to front.