The Horn, often called the French Horn, is thought by many to be the most beautiful sounding instrument in the orchestra. Its mellow sound can turn a simple tune into something that both soothes and lifts the spirit. They can also be tremendously powerful and when the whole section plays loudly the sound will break through any orchestral texture.
Before the 20th century most people recognised the horn's strong relationship with the hunting horns of the past and composers frequently used wrote rustic hunting style music for the horns. However in our modern, urbanised times the horn has lost its rural connotations for us.
Perhaps the most common use of the horn is simply as harmonic filling - somewhere between the bass line and the melody. Horns are perfect for holding long, sustained notes discretely in the background above which melodies can float, around which accompaniments weave and beneath which bass lines wander. This use of the horn is one of the key orchestral techniques that composers learn early on. This is not simply because it's one of the great ways of tying the orchestra together to create a unified sound, but because it's very easy to do and sounds fantastic.