The most common point of failure for your vehicle's horn is usually corrosion or a blown fuse. It's rarer, but sometimes it can break inside or simply wear out. It operates using a magnet and a contactor that interrupts the electrical signal to the magnet very rapidly causing the magnet to vibrate. This in turn causes the high-pitched "humming" sound. Most often a simple changing of the fuse fixes the issue. Sometimes, however, the magnet or contactor inside can stop vibrating, either by corrosion of the metal changing the pitch, or damage due to said corrosion. It's a good idea to start by testing the fuse and checking the wiring leading up to the horn to be sure there isn't a disconnect. This is a much cheaper fix. After you determine it's the horn itself, it's a good idea to get it changed right away. Even though you may not use it often, it's still a required component in your vehicle by law and an officer can write you a ticket if it's not working properly.
Horns are typically made with various metal components. The diaphragm is made with flexible "spring" steel, and there's a magnet inside with other smaller metal components. A contactor interrupts the electrical signal extremely rapidly which causes the magnet to vibrate against the diaphragm. This vibration is the sound you hear. Automobiles have been using different styles of horns since their inception, and they're used in other forms of transportation too, such as bicycles. Their purpose is get the attention of someone not paying attention or doing something they shouldn't be, like driving on the wrong side of the road, trying to turn into an exit only, etc.
Having an improperly working horn or one that isn't working at all is something a police officer can write you a ticket for. It's a good idea to get it replaced as soon as possible to avoid that. Car Parts Discount has horns in multiple brands available for a wide variety of vehicles. We also carry a few custom style ones for use in a project car. To see if we carry them for your vehicle, simply enter your vehicle information above and look in the Electrical category under the Horns subcategory.