Saturn
Saturn is very similar to Jupiter in its basic structure, except it is smaller. It has even more moons than Jupiter, as well the well-known ring system. In the last decade or so, Saturn and its moons have been studied by the Cassini spacecraft. It has sent back lots of magnificent pictures as well as most of the scientific data we have on the Saturn system.
The rings of Saturn
Saturn has an impressive ring system orbiting it. The rings consist of a huge number of smaller debris, mostly water ice, ranging in size from around ten metres to tiny dust particles. Various moons of Saturn affect the shape and distribution of the rings with their gravitational and tidal forces. The rings start about 6000 kilometres above the top of Saturn's atmosphere and are almost 120000 kilometres wide. The rings are very thin, possible only one layer of particles, meaning that they might be as thin as some tens of metres. More recently, it has been discovered that all of the giant planets have a ring system. However, Saturn's is the only one that is large and dense enough to be clearly visible.
Titan
Titan is Saturn's largest moon. It is the largest moon in the solar system, being even larger than our Moon.
Titan has a thick atmosphere of mostly nitrogen, very much like the Earth. The atmosphere is even thicker than the Earth's, with the surface pressure roughly twice Earth's sea level air pressure.
Titan is much colder than the Earth, however. It has an active liquid cycle very much like the Earth's, the only other place we know. But instead of water, Titan has lakes and rivers of liquid methane, which evaporates and forms clouds and then rains back down again.
The Cassini spacecraft released a lander called Huygens onto Titan. This is the source of much of our information on the surface conditions.
Uranus and Neptune
Uranus and Neptune were the first solar system objects discovered in recent memory. Before the discovery of Uranus, the solar system outside of Earth's orbit consisted of Mercury, Venus, Mars, Jupiter and Saturn. The two remain the least well known planets, as they have not been visited by any spacecraft on purpose. The Voyager probes flew past the two and provided most of the information we have on them.
Uranus and Neptune are so called ice giants. They are somewhat smaller in size and mass than Jupiter and Saturn, and their composition is quite different. Instead of being mostly hydrogen gas, these planets consist of "ices", mostly water and methane. The name "ice giant" is slightly confusing as these molecules are actually mostly not in ice (solid) form on the planets. These chemicals give the two planets their strikingly blue color.
Like the gas giants, both Uranus and Neptune are orbited by large number of moons, many of which seem very interesting, but they have never been studied very closely.
Dwarf planets
A recent new class of solar system bodies is the dwarf planet. The classification was created after many objects were discovered in the same region as Pluto, which are similar (or even larger) in size to Pluto.
A dwarf planet is defined as a body that is large enough to be spherical (the gravity of a large enough body will pull it into spherical shape) but not massive enough to be the gravitationally dominant body in its orbital region.
The only dwarf planet in the inner solar system is Ceres, but outside of Neptune's orbit there are already several (nine, to be precise) besides Pluto. It is estimated that there may be hundreds or thousands of them, but they are still very difficult to find with current telescopes.