What exactly is a meteorite? What is it made up of?

Answer

First some correct terminology: a meteoroid is a small piece of matter that ends up in the atmosphere of the earth. The light phenomenon that one sees is called a meteor. If a remnant reaches the surface of the earth, you have a meteorite.
Most meteoroids have a mass of less than a gram. They move around the sun in their own orbit, so they can be attracted to the earth when they come close. The maximum speed of a meteoroid when it comes close to the earth is about 42 km/s. If it happens to hit the earth head-on, it will plunge into the atmosphere at a speed of 72 km/s (the earth itself has an orbital speed of 30 km/s). The lowest speed at which a meteoroid enters the atmosphere is 11 km/s .
Between roughly 140 and 70 km altitude, a visible flash of light can occur. Most meteoroids vaporize completely as they pass through the atmosphere, while the larger ones can yield a meteorite. But even the lightest meteoroids survive their journey because they can radiate their frictional heat fast enough and thus gradually slow down, after which they slowly drift down to the earth’s surface.
For the composition of meteoroids refer to the following link :

http://en.wikipedia.org/wiki/Meteorite

scroll down to the heading “meteorite types”

Be sure to check it out

http://en.wikipedia.org/wiki/Meteor_shower

for the important connection between comets and meteor showers such as the famous Perseids in the month of August.