Here we go. This is from allmusic.com, my favorite music reference site:
http://www.allmusic.com/cg/amg.dll?p=amg&sql=77:217Hard rock generally prizes big, stadium-ready guitar riffs, anthemic choruses, and stomping, swaggering backbeats; its goals are usually (though not universally) commercial, and it's nearly always saturated with machismo. With some bands, it can be difficult to tell where the dividing line between hard rock and heavy metal falls, but the basic distinction is that ever since Black Sabbath, metal tends to be darker and more menacing, while hard rock (for the most part) has remained exuberant, chest-thumping party music. Additionally, while metal riffs often function as stand-alone melodies, hard rock riffs tend to outline chord progressions in their hooks, making for looser, more elastic jams should the band decide to stretch out instrumentally.
They are rooted in the same types of music (The Kinks, Cream, Jimi Hendrix) but later split off. Does that help?