The horn is a brass instrument consisting of about 12–13 feet (3.7–4.0 m) of tubing wrapped into a coil with a flared bell. A musician who plays the horn is called a horn player (or less frequently, a hornist).

Descended from the natural horn, the instrument is often informally known as the French horn. However, this is technically incorrect since the instrument is not French in origin, but German. Therefore, the International Horn Society has recommended since 1971 that the instrument be simply called the horn. French horn is still the most commonly used name for the instrument in the United States.

Horns have valves, operated with the left hand, to route the air into extra tubing to change the pitch. Most horns have lever-operated rotary valves, but some horns like the Vienna horn use piston valves (similar to trumpet valves). A horn without valves is known as a natural horn, changing pitch along the natural harmonics of the instrument (similar to a bugle), but with a wide range of notes due to the long tubing, coupled with a small mouthpiece, which causes the playable range of the horn to be higher in the harmonic series, where pitches are closer together.

Three valves control the flow of air in the single horn, which is tuned to F or less commonly, B♭. The more common double horn has a fourth valve, usually operated by the thumb, which routes the air to one set of tubing tuned to F or the second set of tubing tuned to B♭. Triple horns with five valves are also made, tuned in F, B♭, and a descant E♭ or F. Also common are decant doubles, which typically provide B♭ and Alto F branches. This configuration provides a high-range horn while avoiding the additional complexity and weight of a triple.