Frequently, a formidably large chunk of space rock zooms past Earth, unseen until it is far too late. These random agents of chaos are the first thing that comes to mind when you think of asteroids. Surprisingly, however, most asteroids spend their time loafing around in a huge ring about the sun.
When the solar system was young, individual planets formed out of something called an accretion disc
. This is a flat circle of material surrounding a new star. Nearly all of the matter that will eventually form a solar system is sucked in to the center to form the star, but a little bit of it is left further away. Within this swirling mass of gas and dust, small chunks form. These chunks gradually accrete and begin to draw other chunks in through gravity, forming planets.
A particularly lucky chunk eventually grew into Jupiter. Jupiter is about two and a half times as massive as everything else in the solar system (except the sun) put together. This mass allows it to dominate the space around it. The part of the disc just inside of Jupiter tried to form planets too, but whenever Jupiter passed, its pull caused turbulence amongst the forming planets, smashing them together and shattering them into small pieces. Some of these pieces fly away into space never to be seen again, but many remain in orbit in a giant rotating cloud.
There are enough asteroids that all added together, they would weigh roughly 4% as much as the moon. However, they occupy such a large region of space that they are spread out very thinly. Spacecraft such as Pioneer and Voyager have easily traveled entirely through without hitting anything; the belt is not thick like a Star Wars movie depicts
Having so many asteroids floating around should be bad for us living things, right? According to the speculation of those astronomers, this may not be the case. Some asteroids carry large amounts of water ice and organic compounds (from which life as we know it is primarily built). These asteroids are periodically tossed out of the belt by Jupiter's passing or colliding with one another. Many of these smash into a young, unsettled planet such as earth, depositing cargoes of water and chemicals which may help set the stage for life.
Further, occasional (we hope!) catastrophic asteroid collisions may help "shake things up", keeping one set of life forms from perpetually dominating and allowing evolution to stagnate. Bad for the dinosaurs, good for mammals like us.