Titan is Saturn's largest moon and the second largest in the solar system (after Ganymede of Jupiter). It is the only moon in the solar system with clouds and a dense, planet-like atmosphere.

Scientists believe that conditions on Titan are similar to Earth's early years (the main difference is that, because it is closer to the sun, Earth has always been warmer). According to NASA, "In many respects, Saturn's largest moon, Titan, is one of the most Earth-like worlds we have found to date."

More Info: en.wikipedia.org