Hard Rock and Heavy Metal

Heavy metal and hard rock can be very similar, but they are not the same. Heavy metal can be classified as a sub-genre of hard rock because it´s based on it. However, hard rock maintained a bluesy rock and roll identity, and heavy metal took a darker and heavier sound. Lyrics are also different; hard rock lyrics are about masculinity, drugs, alcohol and sex, while heavy metal lyrics are related to heaven, hell, Satan, demons, fictional characters and church. Vocals are different too; hard rock voices are nowhere near as screaming as heavy metal voices are. 

Smoke on the water, by Deep Purple, example
of Hard Rock.

Hellrider, by Judas Priest,
example of Heavy Metal.