Ambient and drone music exists on the outskirt of popular music. The former genre, which gained popularity in the late '60s and early '70s due to the abundance of synthesizers, utilizes repetitive sonic patterns to evoke something soothing, unobtrusive, and absolutely beautiful. A typical ambient track might not have much in the way of melody or harmony, but the textures of the synthesizers and the general "feel" of the track is enough to transport the listener to another dimension. The latter genre pretty much does what it says on the tin, a musician will drone on one note for a very, very long period of time, something that eventually leaves the listener completely hypnotized and entranced. This genre hasn't quite experienced the same amount of mainstream success as ambient, you don't see bands like AIR and Cigarettes After Sex popularizing "drone pop", but it's still an important genre nonetheless. Neither ambient or drone are terribly easy genres to get into, so consider this a primer on both of them.