The Great Highland Bagpipes, or GHB, are operated by filling a bag with air and forcing the air through several reeds. This contrasts with instruments like a clarinet or a saxophone where the player actually places his or her mouth on the reed.
The GHB also uses two different types of reeds. First are the single reeds used in the drones. These work somewhat like a saxophone in that they are a single piece of material that vibrates against a solid. the GHB pipes generally have three of these single reeded drones.
The most important reed however, sits in the chanter and is a double reed like that of an oboe or bassoon. This reed consists of two flat pieces of cane or plastic that vibrate against each other when air is passed through the reed.
In order to pass air through the reeds, the piper needs to maintain a rather high pressure of air which is where the bag comes in. Once filled with air, the piper merely squeezes the bag and/or blows new air into the bag and when a certain pressure is reached, air flows out through the reeds creating the familiar bagpipe sound.
So the piper blows air into the bag via a blowpipe that only allows air to pass one direction, the air fills the bag, and when the right pressure is reached – the music starts. A really simple instrument.