Asteroids are minor planets that circle the sun. Asteroids can be made of rocks, metals or the mixture of both.
C-types (carbonaceous) are asteroids that made up of rock, and are the most common ones. They are usually dark coloured and difficult to spot. On the other hand, M-types are asteroids that contain only metal. They are the rarest and brightest. S-types (silicaceous) are the asteroids that contain rock and metal. They are lighter in color comparing to C-types.
Most asteroids circle the sun in the asteroid belt, a broad band between Mars and Jupiter while some travel outside the belt, moving towards Earth. There are millions of asteroids, and the mass of all of them adding together is less than half of the moon. The biggest asteroid ever discovered is Ceres. The diameter of Ceres is about 930 kilometers.
For more facts about asteroids, you can visit the following links:
www.solcomhouse.com/asteroids.htm
en.wikipedia.org/wiki/Asteroid
www.solarviews.com/eng/asteroid.htm
www.bbc.co.uk/science/space/solarsystem/other_solar_system_b