The use of music was a key component in the fictional Buffyverse established by the TV shows Buffy the Vampire Slayer and Angel.
The “Buffy theme” is the music played alongside the opening credits of the show. The theme itself has no lyrics; it begins with several notes played by an organ, a signifier for horror in movie culture from the 1930s onwards. This is quickly replaced by upbeat rock music; an electric guitar riff signifies youth culture and a post-modern twist on the horror genre.
“They [the Buffy production staff] had hired some famous theme song composer to come up with a theme and they didn’t like the one he came up with so they were kind of on a low budget, and asked local bands to come up with theme song ideas.” 
Nerf Herder later recorded a second version of the theme which was used for the opening titles from season 3 on.