To some, Hard Rock and Heavy Metal are nearly interchangeable terms, since they share so many similarities. In both styles, loud, distorted guitars are prominent and the band is fronted by a lead singer who rarely plays another instrument. However, there's a key difference -- Hard Rock stays true to the bluesy rock & roll of the Stones and has a certain swing in the back beat. Heavy Metal has no swing -- it relies on brutal guitar riffs and pummelling rhythms.
Hard rock evolved in the late '60s, as psychedelia and blues rock began pushing the boundaries of amplification and blues-based riffs. Hard-Rock relies less on improvisation than blues-rock and it isn't as loud as heavy metal, even if it shared the same distorted guitars and long solos.