Image courtesy of NASA

The word asteroid means "star-like". This is maybe the truth for their appearance, since they appear as bright point-like objects, but it surely isn't for what concerns their physical nature. In fact asteroids don't emit light on their own, but are visible only because they reflect sunlight.
So far, several hundred thousand asteroids have been discovered and given provisional designations, and thousands more are discovered each year. The size of asteroids range from dust particles to significant bodies hundreds of miles in diameter (Ceres, the largest observed is 913 km of diameter). Globally, the total mass of all the asteroids is less than that of the Moon.

What are asteroids ?
Asteroids are believed to be the remnants of the material from the circumstellar disk that accompanied the formation of the planets. They therefore represent samples of the primordial solar system. A small portion of the asteroid population is also thought to be composed of burned-out comets whose ices have evaporated away and been blown off into space.

Where can the asteroids be found in the Solar System?
Asteroids can be found in different places in the solar system and the localization of these different groups of asteroids is a good mean to make a first classification (click here to see the map and know more about this classification). Most of them, orbit around the Sun, grouped in the main belt, while others are farther objects, with highly unpredictable orbits, such as the Trojans, which lie on the orbit of Jupiter, or such as the Centaurs, in the very outer solar system. Asteroids that, for some dynamical mechanism, closely approach the Earth are named Near-Earth Asteroids (NEAs) and form a class of particular interest.

Asteroids' physical characteristics and chemical classification
Many different physical characteristics of asteroids are studied (such as dimension, brightness etc) and measured (either by optical and spectroscopic observations). 
Asteroids are interesting objects also from a dynamical point of view, since beside following their orbits, they rotate around a fixed pole, with periods typically between one hour and one day, causing apparent magnitude to vary periodically. Some of them, as Toutatis, however present very strange rotations.
For what concerns their chemical composition, most asteroids are made of rock, but some are composed of metal, mostly nickel and iron. A first classification is made according to their chemical composition (and thus, to their observed spectra) going from the extremely dark, carbon-like C-type to the bright iron-like M-type.