Being a guitar player, I can tell you it all really boils down to the "riff".  A guitar "riff" is basically a pattern of chords or notes that are put together and come off as a really cool sound.  These "riffs" can make or break a song.  Often times they get stuck in our heads and become unforgetable. The website Spinner has come up with a list of the 50 greatest guitar riffs. I don't agree with everything on the list, but they do have some great ones listed.