An asteroid is any one of many rocky, metallic bodies that orbit the Sun but have no atmosphere and are too small to be classed as planets.
The main asteroid belt lies between the orbits of Mars and Jupiter, and comprises tens of thousands of irregularly-shaped bodies, from pebble-sized asteroids to the giant Ceres, which is also classified as a dwarf planet. The mean diameter of Ceres is 940 km, or about 7.4% as large as Earth's.
The Kuiper belt, beyond the orbit of the Planet Neptune, is also believed to contain tens of thousands of asteroids and dwarf planets.
While other primordial material in the early Solar System clumped together to form planets, it is likely that Jupiter's strong gravity prevented the asteroid belt forming into new worlds.