Megalodon, Carcharocles megalodon, was the largest shark to have ever lived.
They may have grown up to about 50 feet (15 meters), about the same size as a Humpback Whale, twice the size of an Orca. Some paleontologists, though, doubt they grew longer than about 33 feet (11 meters) long.
Though most people picture them looking like the Great White Shark of today, they may have been much slimmer of build. Their closest living relative is the Blue Shark, which is a much slimmer-bodied shark than the Great White Shark.
Shark skeletons are made mostly of cartilage rather than bone. Cartlidge is the same stuff that makes up the “nose bone” and your external ears. It does not fossilize well, and shark fossils other than teeth are extremely rare. However, sharks replace their teeth constantly, regularly shedding them, and each animal may go through thousands of teeth in a lifetime. Shark teeth are extremely hard, like our own, and are built of bone and enamel. They last a long time even when shed, and fossilize well.
Megalodon teeth have been found around the world wherever there were ropic or subtropic waters while they were alive. They are quite common fossils. Sharks teeth in general are one of the most common fossils found.
Great White Sharks of today only grow to 21 feet (7 meters) at the most, most quite a bit smaller. Whale Sharks, the largest living fish grow to about 33 feet (11 meters).
Megalodons are most definitely extinct, despite science fiction movies trying to say otherwise. There have been no Megalodon teeth found younger than about 2.6 million years old, which is older than the human species.