Some call them 'minor planets' and others call them 'planetoids'. Most of them are in abroad orbit between planet Mars and Jupiter. Most asteroids have highly irregular shapes and are composed of nickel, iron and stone. Asteroids look like the moons of Mars - small, potato shaped and cratered.
The first asteroid was discovered on January 1,1801 by a Sicilian astronomer, Giuseppe Piazzi. He found something which he thought was a missing planet, but it turned out to be a mere pebble of a planet only about 800 km across, 2/9 the the size of the moon. This asteroid later got named as Ceres. Then in 1802, another small planet i.e. asteroid Pallas was discovered in the gap between Mars and Jupiter. It was smaller than Ceres. In 1807, the fourth object Vesta, the brightest and the only asteroid that can barely be spotted with the naked eye, was discovered. By 1890 astronomers had found about 300 of these asteroids. Today, we know the orbit of more than 200 asteroids. It is said that there are about 5 lakh asteroids in the Solar System. Majority of asteroids are found between Mars and Jupiter. Their orbits are grouped into bands separated by so-called Kirkwood Gaps, caused by the influence of Jupiter's gravitational field.

Image: Asteroid
Source: NASA/NSSDC

The average time an asteroid takes to go around the Sun, its revolution period is about 5 Earth Years. A few have long stretched out orbits, taking them close to the Sun and the Earth. Sometimes they are prone to collide with the Earth, but if the asteroid is comparatively smaller, it will be burned down in the atmosphere to give rise to what we call a Shooting Star. If the asteroid is big in size, it will not be completely burned down in the atmosphere, a hot residue may hit the surface causing a crater.