The outer four planets of the solar system are known as the giant planets, because of their large size (compared to the terrestial planets).
The giant planets of our solar system can be further divided into two categories: gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune). The gas giants consist mainly of hydrogen (up to 90%) and helium, while the ice giants are mostly "ices" such as water and ammonia, with only some 20% of hydrogen.
Jupiter is by far the largest of the planets. Its mass is 320 Earth masses, and it's three times heavier than the second largest planet, Saturn. Compared to the Sun, it is still small. The Sun is over a thousand times heavier.
Jupiter consists probably of a rocky core maybe the size of Earth, deep inside the planet, surrounded by a layer of mostly hydrogen and helium over fifty thousand kilometres thick. The top 5000 kilometres or so form a gaseous atmosphere, under this the pressure is high enough for the gases to turn into liquids, and even deeper, around the core, the liquid hydrogen is compressed so much that it becomes "metallic" in its electrical properties.
Jupiter has dozens of moons orbiting it. Most of these are small, and are probably captured asteroids, but there are four particularly large and interesting moons. These were first discovered by Galileo within his first year of telescope observations and are collectively now known as the Galilean moons.
Io is strikingly yellow in colour. It is the most volcanically active body in the solar system, due to strong tidal effects of Jupiter, which deform and heat up the crust.
Europa's surface is covered in ice. The surface is very smooth, and shows long reddish crack lines. Under the ice may be a liquid ocean or a layer of icy slush, surrounding a rocky core. The ice surface shows evidence of a process similar to Earth's plate tectonics. The surface has very little cratering. An old surface would have accumulated many more craters from asteroid and meteor impacts, so some process must renew the surface fast enough to erase the cratering record.
The possibility of a liquid ocean makes Europa very interesting for the search of life outside Earth. All life as we know it on Earth requires liquid water to thrive, so we look for signs of water first, when we look for life. The reddish areas at the cracks in the ice are interesting targets for future space missions, because the reddish material is likely to have brought up from under the ice by water flowing up to the surface through the cracks.
Callisto's surface looks rocky and heavily cratered. The moon seems to consist of a mixture of mineral and icy material. The cratering of the surface shows that no active processes have changed the surface in a long time.
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.
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 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 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 consists 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 colour.
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.