Titan is the largest moon of Saturn, the second-largest in the solar system. (Once is was thought that Titan was the largest, but Ganymede won out.) Titan is interesting because it has a thick, nitrogen-based atmosphere (containing more mass than Earth's atmosphere!) Unfortunately, because of the clouds no one can see the surface, except by infra-red imaging (which is being done with the Keck telescope and others). Because of the atmosphere, pools or lakes of liquid ethane/methane might be stable on Titans surface, which would make it a great natural laboratory of the pre-biotic chemistry that may have led to the creation of life itself. In 2004, the spacecraft Cassini will drop the Huygens probe into Titan's atmosphere and send up data from the surface. That'll be VERY interesting. For now, if you want a very hard-science view of what Titan might be like, check out Stephen Baxter's novel "Titan".