osu! is a rhythm game. It's as simple as that. While the main feature of the game might be it's resemblance to the Ouendan or Elite Beat Agents(EBA), there are two other modes, Taiko, and Catch the Beat.
The game allows players to play along to certain songs in the form of "beatmaps." Players will need to download these from the osu! website. There are TONS of different beatmaps available, including anime songs, video game tunes, and other famous artists. Search for them. If players don't find a particular beatmap available of a certain song, the game allows players to create their own!
Source of Review
I like this game a lot. The fact that you can play online and compete against other players blow my mind. I also like the fact that you can compete even when playing solo. There is a worldwide ranking system where you can compete and so who does the best in that certain Beatmap. I love this game and deeply recommend at least trying it since it's free.