Asteroid Belt
The asteroid belt is the region between the orbits of Mars and Jupiter, about 2.2 to 3.2 AU from the Sun, where most asteroids orbit.
Definition
The asteroid belt is a wide ring of rocky objects circling the Sun between Mars and Jupiter, separating the four inner rocky planets from the four outer giants and holding millions of asteroids of all sizes. The main belt spans roughly $2.2$ to $3.2$ AU and contains over a million known asteroids, yet their total mass is only about $3\%$ of the Moon's, with Ceres alone holding about a third of it. It is a population of remnant planetesimals whose growth into a planet was halted by Jupiter's gravitational stirring, which raised collision speeds above the threshold for accretion. Its structure is sculpted by mean-motion resonances with Jupiter that open the Kirkwood gaps at semi-major axes such as $2.5$ AU (the $3{:}1$ resonance) and $2.82$ AU (the $5{:}2$ resonance); bodies drifting into these gaps have their eccentricities pumped up and are delivered to the inner solar system as near-Earth asteroids and meteorites.
Example
Ceres, the belt's biggest object, is a dwarf planet, and NASA's Dawn spacecraft visited both it and the large asteroid Vesta. A spacecraft traveling from Mars at $1.5$ AU to Jupiter at $5.2$ AU crosses the whole belt, and every outer-planet probe from Pioneer 10 onward has done so safely. By Kepler's third law an asteroid at $2.5$ AU has a period of $2.5^{1.5} \approx 3.95$ years, exactly one third of Jupiter's $11.86$ years, so repeated tugs at the same orbital phase clear that zone within a few million years.
Key Insight
Even with millions of rocks the belt is mostly empty; standing on one asteroid you probably could not see another with your eyes. The belt is not a shattered planet: there was never enough material there to build one, and Jupiter's gravity stirred up what there was so pieces broke apart instead of sticking. Its composition changes with distance, from metallic and stony asteroids in the inner belt to dark carbon-rich, water-bearing asteroids in the outer belt, preserving the temperature gradient of the original solar nebula.