osu! is primarily played using a mouse to click 'beats' displayed on-screen in time with the music. This is analogous to tapping the beats on the Nintendo DS screen with a stylus. At the top of the screen, a health bar is present, which is constantly drained. If the player hits or clears a beat, the health meter at the top of the screen fills slightly. If he misses a beat, a large part of the meter is drained, and if it gets completely empty, he fails the song. There are three types of beats in the game: Hit circles, sliders and spinners. Every time the player hits a hit circle or an end circle, touch a slider tick, or clear a spinner, the combo is raised by 1. If he misses any of the above, the combo is reset back to 0.

osu! is now beyond its time in many areas, with a huge focus on web integration and community aspects. Features are added based on people's requests, and their perceived usefulness to the community as a whole. The map submission process (Beatmap Submission System) is seamless and intuitive from start to finish. Importing songs, loading skins and playing the game is simple enough for almost anyone to handle.
Multiplayer support is available. New gameplay modes are implemented to expand the scope of this game beyond its original expectations.

I hope that's enough information, sadly I couldn't find a wikipedia page or more info about this game.

This game is REALLY hard, as it involves fast clicking and mouse movement.
But, it is fun.