Aren't asteroids, meteors, and meteorites all the same thing? Almost, but not really.
An asteroid is a rocky object in space that's smaller than a planet. Some are so big they're almost planets.
Ceres, for example, is the largest asteroid in the solar system. It's smaller than the moon, but big enough to be nice and round and have several layers inside. Some scientists believe Ceres has a thin atmosphere, and its designation has changed through the years. Sometimes it's a planet, sometimes it's a dwarf planet, sometimes it's an asteroid.
Some people classify asteroids depending on size and where they are, but for now, let's settle for it being a chunk of rock. =)
A meteorite is a meteor that survives plunging through the atmosphere and lands on the surface. Most are tiny, but some are so big they change the face of the world. Close to where I used to live in Sweden is a large lake that stems from a meteorite hitting us.
I found this great overview on the American Meteor Society's website.