Meteorites are basically rocks from space.
After traveling around in space for usually a long time - often hundreds of millions of years, they hit the earths atmosphere and survive the trip to the surface. When in space the rock is called a meteoroid or an asteroid. When in the earths atmosphere it is a meteor and sometimes is seen as a fireball. If the rock survives burning up in the earths atmosphere and lands safely on the surface it is called a meteorite. About 18,000 rocks hit the earths atmosphere every day.