What's the Difference Between Hard Rock and Heavy Metal?

I keep thinking to myself, what is the difference between Hark Rock and Heavy Metal? Hard Rock as we know it is obviously the harder heavier version of rock, while Metal is more of that raised to the nth degree. The question is, what distinguishes one form of heavy music from the other?

Differentiation in heavy music can be easily separated by extremes. Such has Def Leppard vs. Slayer. Slayer can definitely be defined as Heavy Metal while Leppard as Hard Rock. What happens when artist’s crossover? Or when artists have variations in their styles? What is the fine line in the sand that would be drawn to distinguish what category a band would fall into, Heavy Metal or Hard Rock? 

Alice Cooper, if you look at the original Alice Cooper band, they are discerned as classic rock with hits like “No More Mr. Nice Guy”, “I’m Eighteen” and “Schools Out”. Those songs compared to other music of that time can be defined as Hard Rock. As you go to later in his career, such as the album Brutal Planet, that has a much heavier heavy metal sound. So, does that make Alice Cooper Hard Rock, Classic Rock, or Metal? 

When comparing bands from the same era, they can have total different sounds from each other even if they come from a similar background. Bands are affected by their influences and tastes in music which evolve over time. So there really isn’t a fine line for determining which band is Hard Rock and which one is Metal. The difference between Hark Rock and Heavy Metal bands, I think it comes down to perception and attitude. It’s the impression left on the audience as portrayed by the artists that make them Hard Rock or Metal. I prefer not to label bands and their music. I believe a great song is a great song no matter what genre. The band and their song(s) will stand the test of time.