"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."
-from official website
By the way, if you've ever played elite beat agents, you know what this game is like. Most (if not all) of the beatmaps (levels) are created by fans, btw. You can also create your own beatmaps.
Screenshots (although if you really want to see what this game is about I suggest you to check on youtube for some gameplay)
(http://img833.imageshack.us/img833/1241/osu5.png) (http://img833.imageshack.us/i/osu5.png/)
(http://img838.imageshack.us/img838/8039/screenhunter01mar221918.gif) (http://img838.imageshack.us/i/screenhunter01mar221918.gif/)
(http://img815.imageshack.us/img815/5596/screenhunter02mar221922.gif) (http://img815.imageshack.us/i/screenhunter02mar221922.gif/)
Download:
http://osu.ppy.sh/p/download