Here's what it sounds like:
It can play a wide range of notes:
The horn has a long ancestry, dating back to ancient times when people signalled to one another by blowing on animal horns. You often hear it called the French horn, which is a bit odd because it didn't originate in France! In film music and classical music, the horn is often cast as the instrument of heroes. It has a huge range of possible notes, and plays an important part in every band or orchestra. Like most of the brass family, the horn has 3 (or sometimes 4) valves, though these are rotary valves rather than the piston valves which are more common in other brass instruments.
Students often ask why you play the horn with the right hand partly inside the bell of the instrument. That's partly to hold it more easily, and partly to give you the ability to shape the sound a bit more by opening and closing the hand slightly, creating different musical effects. In the days before valves were invented, movements of the right hand were also used to help change notes, making it a very difficult instrument to play. Thankfully, we don't have to do that any more! Valves make our job easier.
Here's an introduction to the horn: