Source:
http://en.wikipedia.org/wiki/File:French_horn_front.png
The French horn is a kind of bugle or trumpet - a long pipe of almost 4m in length that is curled up into itself, and with valves. The flare's end diameter is larger than that of the bugle or trumpet. It has valves used to change the length of the pipe.
Its tones are lower than those of the trumpet and cornet, but its range of about 5 octaves much larger. Its mouthpiece is different to the other brass instruments.
The instrument was not invented in France, but in Germany, while it may regarded as an artificial animal horn made of brass, and with valves added.
The acoustic principles of the French horn and English horn (cor anglais) are quite different and should not be confused because of their names.
Source:
http://en.wikipedia.org/wiki/File:French_horn_front.png
http://www.beginband.com/sndclips.shtml