There are several different types of asteroids, identifiable through their composition.
Silicon-rich (S-type) asteroids dominate the inner part of the asteroid belt, closest to the sun. These asteroids are composed of rocky materials -- iron and magnesium silicates -- plus small amounts of metallic iron. About 17 percent of known asteroids are S-type.
M-type (metallic) asteroids that condensed in the middle of the main belt are predominantly metallic iron and nickel, although some of them may have some anhydrous (without water) silicates, hydrated clay minerals, magnetite, and sulfides.
More than 75 percent of known asteroids are C-type (carbonaceous) asteroids, which inhabit the main belt's outer regions. These asteroids are usually composed of organic compounds and hydrated minerals. They are rich in carbon, either in elemental form or in organic matter, such as aliphatic and aromatic hydrocarbons. Any metals in these asteroids are found as silicates, oxides or sulfides rather than in the free form. C-type asteroids that formed beyond the Frost line were able to accumulate ices and hydrated minerals.
Additional websites with information about asteroids, at a variety of levels:
This information is written for a public audience, including older children: Comets vs. Asteroids Fact Sheet (PDF, 1.3 MB)
We recommend you start with Solar System Exploration: Asteroids. This page contains information written at an adult level.
Another starting resource is "The Main Asteroid Belt" by Carolyn Crow.
For information about planetary migration and altering positions of asteroids, check out this press release written by the Southwest Research Institute.
Some asteroids have been compared to enormous rubble piles. For more information on "rubble pile" asteroids, check out Killer Asteroids. Loosely Bound Piles of Trouble.
There are a variety of asteroid articles in Planetary Science Research Discoveries; this educational bulletin is written at a high level for those already familiar with fundamentals of geology:
Discover technological breakthroughs that revolutionized the study of asteroids:
See also the related Year of the Solar System topics: Comets: Small Bodies / Big Impacts and Collisions and Craters in the Solar System: Impacts.
What Are NEOs?
What We Know about NEOs
NEO Questions and Answers
NEO Education and Public Outreach
Big Answers from Small Bodies
Big Answers from Small Bodies: Introduction
Characteristics of Main Asteroid Belt
Implications for Early Solar System History
Dawn Mission to Vesta
Vesta: Impact Flux
Vesta: Water Story