Meteorites are the remains of meteoroids, which are theorized to be parts of asteroids, which are theorized to be parts of planets. Most meteoroids are the size of large grapefruit ranging down to rice grain size. When meteoroids enter Earth's atmosphere, they start to burn as a result of the atmospheric friction. Most burn up completely in the atmosphere, but some do make it all the way to the Earth's surface. When they strike the surface of the Earth, they are called meteorites.
The composition of meteorites is very similar to that of Earth. They can be composed of iron, of stone, or can be a mixture of stone and iron. Some have been found to have deposits of nickel and other elements common to the composition of the Earth.