A horn is an instrument that's made of metal and is played by blowing into it. Another type of horn comes out of an animal's head. Don't confuse the two and imagine a trombone sticking out of a water buffalo's head.
A trumpet is one kind of horn, and another is a silly plastic noisemaker that produces a loud, honking sound. You might blow this horn at a New Year's party or a football game. Your car has a horn too, that you use to warn other drivers of your presence or express your annoyance. The horns that animals have are hard, bony bumps or projections on their heads — you'll see them on goats and bulls, for example.