The French Horn or simply Horn is a brass instrument having a funnel-shaped mouthpiece and long tube piece stretching over 20 feet coiled into a circular shape. Actually its origin is in Germany but it took its final shape and form in France. It is the third highest sounding instrument in the brass family.
There are two types of French Horn instruments – Single Horn and Double Horn. The instrument has lever-operated rotary valves which control the flow of air. Single Horn types generally have three valves and Double Horn type has an additional, fourth valve operated by the Thumb.
Read more about the French Horn here
Here is a basic video on French Horn