Sharks have been around for LONGER than dinosaurs - nearly 400 million years. Some prehistoric sharks were 80 feet long - almost as long as a BASKETBALL COURT.
Sharks started appearing in the Ordovician period over 400 million years ago, before the dinosaurs were around. Later, in the Carboniferous period, there was the "Golden Age of Sharks". Back then there were tons of different varieties of sharks, including sharks with spikes and armor. (Here's Dunkleosteus, an armored shark). Here's another scary pic to give you an idea how big they were.
The biggest shark came around after the dinosaurs were wiped out, about 16 million years ago. We know this shark as "Megalodon", the Mega Shark. "Megalodon" means "big tooth". Here's why. This shark was an estimated 80 feet long and weighed 50 tons - as much as FOUR schoolbuses. Compare that to the Great White, at only 42 feet in length.
For a more realistic depiction of what it would be like to meet a Megalodon, try this video.
(source 1, 2)