Anything particularly LOUD, edgy, heavy, rowdy, or mildly controversial from the 60's or 70's is HARD ROCK. Other characteristics include wild guitar solos or particularly long guitar solos or super long jam songs that chicks didn't usually dig on too much. This is just a simple distinction from all of the other awesome regular rock and pop rock from that time period. Any rock band from the 60's or 70's might have some regular or even mellow rock, or some songs that are definately HARD ROCK songs. Pop rock back then didn't have to be mellow per se. It could also be HARD ROCK. Nothing in the 80's counts as HARD ROCK unless it's by bands leftover from the 60's and 70's or by bands who atleast first became popular in atleast the late 70's and played with the same spirit as the awesome 60's and 70's bands. Van Halen (with Roth) is the epitome of a band that first became popular in the late 70's but then went on to be the premier HARD ROCK band of the 80's that still new what they were doing and played in the true spirit of HARD ROCK. Shit that came later wasn't done properly and at best is just some hair metal or glam metal bands trying to do their impression of hard rock and failing miserably due to their failing metal sensibilities and lack of vocal talent.
HARD ROCK could also be thoughtful or intelligent and not just stuck in rowdy or controversial mode all of the time like how many of today's shoddy imitators like to think of it as being. --- hard rock --> Heartbreaker (Led Zeppelin),Miss Misery (Nazareth),Free For All (Ted Nugent),Stranglehold (Ted Nugent),Back In The Saddle (Aerosmith),Freewill (Rush),Barracuda (Heart),Black Dog (Led Zeppelin),Back from the Storm (Hendrix),
Helter Skelter (Beatles),TNT (ACDC),Susie Q (CCR),
Frankenstein (Edgar Winter Group),Godzilla (Blue Oyster Cult),Cities On Flame (Blue Oyster Cult),Hocus Pocus (Focus),School's Out (Alice Cooper)
-not hard rock, not no way, not no how-> def leppard,gnr and motley crue, uh-uh...forget about it -> motley crue popularized glam/hair metal and gnr was the last popular glam/hair metal band, while def leppard was somewhere in the middle during the full bloom of bloomin' fairy metal.
Learn how to rock with real hard rock which sounds good loud due to superior vocal talent and not like some L.A. pansies screeching like little talentless pussies.