It's basically a delicious clone of "Guitar Hero". That's why we all love the copyright-free world of the internet: if any one greasy nerd has enough time on his hands, he can replicate/steal/crack anything for us all to enjoy.

When you first  download the game, it comes with only 3 songs for you to play around with. But the kicker is that you can get hundreds more RIGHT HERE:  

THE PIRATE BAY ( REQUIRES BITORRENT )
or
KEYBOARDS ON FIRE COMMUNITY ( LIMITED SELECTION, BUT DIRECT DOWNLOADS )

Since the frets are all user-generated and the people who have time to generate web-content are usually tasteless, talentless teenagers, you'll find your good share of terrible nu-metal/ death metal crap and also a decent amount of really badly-fretted songs that will make you want to smash your keyboard to bits.  Oh, and for some reason, a lot of spanish power metal. But mostly, they're well-done and enjoyable.

And you  can also get every song from Guitar Hero 1 and 2! 

BUT BEWARE, READ THIS BEFORE PLAYING THE GAME: ===  Go in the "options" menu, then "game settings" and enable the "tappable keys" options, this will let you simply press down on a full note ( if you're hit another one before it ) to "hit" it, otherise the game is impossible in many parts. Once gain in case you're slow: In this game, you have to press both which ever key comes up AND enter AT THE SAME TIME. But when you have notes with a little dot in them, you don't need to hit ENTER! But you do need to lift your finger and HIT THE NOTE. ===  You can also reconfigure your keyboard!! You don't have to hold it like a guitar if you have a shitty weirdo-space-age-dildo keyboard like I have. Just set the frets like so : fret 1 == z fret 2 == x fret 3 == c fret 4 == v fret 5 == b OR YOU CAN PLAY THE CLASSIC WAY === Some keyboards don't allow certain key combinations, and this will PISS YOU OFF ENDLESSLY, for instance, some songs will require you to press " fret 1 + fret 4" but maybe your keyboard doesn't register " z + v + enter " so you're fucked. Buy another keyboard. With mine, I can hold down all 5 notes and press enter, and it still works. === You can play the game like so : fret 1 == you have to press ONLY z fret 2 == you CAN press Z + X and it still works ! and so on : fret 1 ( green ) = Z fret 2 ( red ) = ZX fret 3 ( yellow ) = ZCX fret 4 (blue) = ZXCV fret 5 ( pink )= ZXCVB But this doesn't work the other way around!!   Also, note that this works for power chords, i.e. pressing two keys at once. In Guitar Hero, when you have FRET 1 + FRET 4, you have to hit ONLY those two, but in this game, you can hit FRETS 1-4 and it still works. BUT ONLY IF YOUR KEYBOARD ALLOWS YOU TO PRESS DOWN THAT MANY BUTTONS!!! This makes the game much easier ( which is of course the point). And note that some songs require you to hold down 3 buttons at once, and a lot of keyboards have a limit of 2, so you'll hate the assholes who fretted those songs if you happen to have a bronze-age keyboard.