Hard Rock

metallica etc.)

I would consider hard rock to be heavier rock music that could be (but doesn't have to be) based in blues and should actually ROCK when you listen to it. As oppose to heavy metal which kind of emphasises heaviness over rockiness. So AC/DC rock when they play their heavyish music thus are hard rock. Black Sabbath emphasise heaviness so are metal despite being rooted in blues. Metallica go for speed and heaviness so a well into the metal category, with some nods to the hard rock genre (they do still rock, at least more of their 90s stuff does).
And waaay Alice Cooper! Definitely hard rock. Definitely awesome.

Iron Maiden

To the addition of AC/DC, Motorhead, Alice Cooper, I'd add Queen, Led Zeppelin, Deep Purple, The Doors and Uriah Heep.

(Iron Maiden - hard rock!? Blasphemy!!! )

Motorhead!

In addition to those mentioned by others: Silvertide, Stone Sour, Black Stone Cherry, old Aerosmith, Van Halen (with Dave), Guns n Roses, Skid Row, The Four Horsemen... I could go on, but I won't.