NASA Image from Ceres site
Asteroids are small rocky bodies, most of which orbit the Sun between Mars and Jupiter. Their orbits are typically more eccentric than those of the planets. The largest asteroid, Ceres, is 940 kilometers in diameter and has about 1/10000 the mass of the Earth. With over 7000 asteroids catalogued, the total number may exceed 100,000. However, their combined mass is less than 1/10 that of the Earth's Moon. Ceres and the second-largest asteroid, Vesta, were visited by the Dawn spacecraft.
The vast majority of the meteorites that hit the Earth are thought to come from the asteroid belt.
Asteroids are classified from their observed reflectivity, with C-type asteroids making up about 75% of them. C-type or carbonaceous asteroids are dark from the high content of carbon in them. S-type asteroids contain silicate and are more reflective - they make up about 15% of the asteroids. Most of the remaining asteroids are called M-type and contain large fractions of iron and nickel. The C-type asteroids are supposed to be very primitive material that has not been significantly heated or changed chemically since they were formed some 4.6 billion years ago.
Chapter 9 of Planetary Sciences by de Pater and Lissauer is a detailed treatment of asteroids.
Solar System Illustration
Solar System Concepts
Chaisson and McMillan