Asteroids are a class of small Solar System objects that orbit the Sun.  There are more than million known asteroids, many thought to be the shattered remnants of planetasimals, bodies within the young Sun's solar neighborhood that never grew large enough to become planets.  LSST expects to discover about 10 million asteroids during its ten-year survey lifetime.  For more information, check Wikipedia or the Teach Astronomy's Asteroid pages.