The French horn, or horn, is a brass instrument that consists of about 12 feet of coiled metal tubing. Sound is created by vibrating the lips against the mouthpiece. A single horn, which is in the key of F, has three valves. A double horn, which can play in the key of F or B flat, has four valves. The extra thumb valve is used to choose between the F horn and the B flat horn.
For more information about the horn, try these sources:
Can you hear the french horn in this piece?
Rhapsody in Blue by George Gershwin