Guitar Hero is a game in which you can play at being a guitarist. Think DDR but with a cheap plastic instrument in your hands of questionable durability.
The game owes its success largely to the incredibly popular song "Jukebox Hero" by Foreigner, as it clearly takes its name and general theme from it. You can easily tell this by setting Jukebox Hero to repeat and them playing the game with the TV muted.
See, it's sort of like how you can play The Dark Side of the Moon while watching The Wizard of Oz...But it's cooler, because I mean come on, it's Foreigner. (It'll even make you do better in the game. I promise!)
So, any other Guitar Heroes out there? Guitar Hero 3, as of this post, is coming out pretty soon. Really looking forward to online play.