Saturn's rings are mostly made of water ice, mixed with a small amount of rocky debris and dust. We know this because we've been there! Saturn has been visited by four robotic spacecraft, but most recently by the Cassini probe. (The Cassini probe has been orbiting Saturn and returning information since it arrived in 2004.)

Saturn is the second largest planet in the solar system, approximately 95 times larger than earth, and is composed mostly of hydrogen and helium around a nickel-iron core.

The Huygens lander that came with the Cassini probe has actually landed on Saturn's moon Titan. Titan is the second largest moon in the Solar System - larger even than the planet Mercury - and is itself composed mostly of water ice!

You may notice that I use the term water ice to refer to water in its solid state. This is because there are different kinds of ice, and it's best to be specific in science.

More Info: en.wikipedia.org