There is a belt between Mars and Jupiter because the gravity of newly formed Jupiter brought an end to the formation of planetary bodies in this region and cause the small bodies to collide with one another, fragmenting them into asteroids.
Ceres is the largest belt at 590 miles and almost became a planet. It is about 25 percent of the asteroids total mass and was discovered in 1801 by Guiseppe Piazzi.
Fun Facts: - They're all made of different minerals/substances - It was believed by many scientists than an asteroid impact was the cause behind the extinction of dinosaurs.