When the Solar System formed, not everything was used to make planets and moons, leftovers became asteroids and comets. They are perhaps the oldest remnants of the early Solar System and what we learn from them tells us about our own early history.
Most of the asteroids lie between the orbits of Mars and Jupiter, but some have very elliptical orbits which bring them very close to Earth. They range in size from about 30% the diameter of our moon to pebble sized objects. In number, there are probably billion of asteroids in the solar system.
The largest known asteroid is Ceres which was the first asteroid to be discovered in 1801. Ceres is about 1/3 the size of our moon. It can be seen in amateur telescopes if one knows where and when to look.
Most asteroids are of the small irregular variety. None of them are large enough to have high enough gravity to pull themselves into spherical shape. Images of the same asteroid vary in brightness which confirms the irregular shape.
The Galileo spacecraft passed through the asteroid belt twice on its way to Jupiter. The spacecraft passed only 16.000 km from the asteroid Gaspra in 1990 and returned excellent images. Gaspra is an oblong world measuring 20 by 12 by 11 km. It is covered by a layer of shattered soil about a meter deep. There are also numerous craters on Gaspra's surface.
In 1993, Galileo passed only 3500 km from the asteroid Ida. This was another irregularly shaped world about 52 km long. Ida is also a heavily cratered world. What was startling about Ida was that it was found to have a smaller body orbiting it! The little moon was 1.5 km in diameter, the first found to orbit an asteroid.
The asteroids which pass close to Earth are known as Near Earth Objects. They are tracked by a program called Spacewatch. The program has found over 100 times more near Earth objects than was expected. Over 50 of these objects pass within the moon's orbit each day. The danger of a collision with Earth is minimal but still significant. Astronomers are more interested in the origin of these objects.
Astronomers have divided asteroids into 3 classes, according to color and spectra. S type asteroids, including Gaspra, are reddish in color. It is believed these are silicates mixed with metals. C type asteroids are very dark in color and appear to be carbonaceous in composition. M type asteroids are bright but not reddish. These appear to be iron-nickel mixtures. S types are common in the inner asteroid belts while C types are common in the outer belts.