Asteroids are rocky and metallic objects that orbit the sun. They can be as small as a pebble or as large as 1000km in diameter. They are believed to be the leftovers from the formation of the solar system although some believe they may be the remains of a planet that was destroyed in a massive collision many millions of years ago.
When an asteroid is on a collision course with Earth it's called a meteoroid and should one enter our atmosphere, friction causes it to incinerate into a streak of light which is known as a meteor. If the meteoroid does not burn up completely the part that's left which subsequently crashes into the surface of Earth is called a meteorite.