Classic rock is one of the most popular styles of music in the world, touching the lives of generations of people. Yet ironically, no one knows exactly what classic rock is. Led Zeppelin, Jimi Hendrix, Deep Purple: There are certain artists everyone agrees fit the classic rock bill, but where exactly does classic rock begin and where does it end? For some, classic rock constitutes music released during a specific period of time. For others, it's a genre or simply a radio format.