A meteorite is a rock that enters Earth's atmosphere and falls onto Earth, from space. These are fascinating because apart from not being common, they are much older than the rocks we have on Earth. Most of these rocks come from asteroids (which can be thought of very small planets), but other meteorites come from planets (such as Mars or Mercury), from the Moon, or from comets.
The size of meteorites varies greatly. One meteorite for example weighed 60 tons. On the other hand, meteorites the size of grains of sand were discovered as well.
Once a meteorite falls on Earth it is studied and examined in specialised test centres. The testing confirms whether it is a meteorite, and also classifies its type.
Just like fossils tell us a lot about ancient life on Earth, meteorites tell us a lot about the history of our solar system, including how planets and asteroids formed.