(See the Rhea Gallery)
Made of more than two thirds ice and though the rest would be rock Rhea does not have a definite core. The interior is a somewhat homogeneous mixture of rock and ice with the ice nearer the core compressed.
It also appears that Rhea has 3 small rings and is the first moon we've discovered with such a feature.
Rhea's surface is covered with craters and other features with varying reflectiveness. It's very much like a larger and less dense Dione. The moon is also tide locked with the same side facing Saturn. As a result the trailing side is different from the leading side.
See the Rhea gallery here.