The main asteroid belt is found between Mars and Jupiter. It is between 2 and 4 astronomical units from the sun. This means it is two to four times as far from the sun as the Earth is.
The asteroid belt contains a nearly uncountable number of bits of rock left over from the formation of the solar system. Scientists believe that the solar system was formed out of a solar nebula. Rocky debris collided and coalesced to form the terrestrial planets. However, the gravity of Jupiter was so powerful that some of this debris was prevented from forming planets. Most asteroids are either C-type, or carbonaceous; S-type, or silicaceous; or M-type, or metallic. Most asteroids of the C-type are found on the rim of the asteroid belt farthest from the sun.
Most asteroids are very small and have irregular shapes. Other asteroids are large enough for their mass and gravity to pull them into spheres. One of these large, round asteroids is Ceres, which is so big that it's considered a dwarf planet. Asteroids called Trojans also line up in front of and behind both Jupiter and Mars. The former are called Jupiter Trojans, while the latter are called Martian Trojans.