What you're seeing here is an image of Saturn's moon Dione as it passes in front of the moon Titan. The detail is breathtaking, and you can just see the haze of Titan's atmosphere glowing around the curve of the lake-covered moon.
This Cassini spacecraft image was calibrated by Gordon Ugarkovic, who offers this information about the shot:
Cassini NAC RGB view taken 2011-12-07 20:53 UTC.
Distance to Titan: 3.163 million km
Distance to Dione: 1.611 million km
Phase angle: 61 deg
You can see more of Ugarkovic's gorgeous work in his Flickr stream.
Image credit: NASA / JPL / SSI / Gordan Ugarkovic