A meteor is a meteoroid (a small rocky object in space) that has entered the Earth's atmosphere. Friction causes it become burning in the atmosphere, causing the meteor to glow brightly. For this reason, a meteor is often referred to as a "shooting star" or "falling star". Most meteors are small and burn up in less than a second, but some very large ones glow brightly for many seconds and are called "fireballs" or "bolides." (See also Asteroid).
An exceptionally large meteor may not burn up completely in the atmosphere. If part of a meteor hits the ground, it becomes known as a meteorite.
A major source of meteors lies between Mars and Jupiter, an enormous cluster of meteoroids also known as the asteroid belt. Other (minor) meteoroid belts and clouds exist, and when the Earth passes through them periodic meteor showers occur, such as the Perseiids, Geminids, and Taurids.
Earth's orbit is far inside the belt between between Mars and Jupiter; another lies just outside of our solar system. Meteoroids from those belts break free and may collide with Earth.
There are currently several hundred known asteroids, called Apollo asteroids, that are in Earth-crossing orbits.
Meteors are asteroids or pieces of them, which mostly come from one of the two asteroid belts in the Solar System. A large cluster of asteroids lies at the outskirts of our solar system and is known as the Kuiper Belt, another one is the Asteroid belt between Mars and Jupiter.