Beavis and Butthead are cartoon characters, a product of their time, a cultural phenomenon, and a couple of annoying teenagers.
A Phenomenon of Their Time
Beavis and Butthead represented the ethic that MTV were trying to give to their station, whether they truly had it or not. Slacker humour and no-brainer innuendo appealed to a fairly diverse set of groups.
A Cultural Phenomenon
Beavis and Butthead were fairly short lived, but at the height of their fame they span off an album, The Beavis and Butthead Experience, several games including a quiz and one that involved spitting, and a movie, Beavis and Butthead Do America.