Terrain
The planets we know of so far are split easily into two categories - terrestrial planets and gas giants. The first four planets in our solar system - Mercury, Venus, Earth, and Mars - are terrestrial planets, while the last 4 - Jupiter, Saturn, Uranus, and Neptune - are gas giants. Terrestrial planets are more dense than gas giants and, in our solar system, are closest to the sun. They're also smaller, denser, and rockier than the outer planets. Gas giants are planets with deep, massive gas atmospheres and, in our solar system, are farther from the sun. These are the two basic categories we have, though the actual terrain of each planet varies based on the main components of each planet. Mercury is, of course, rocky, with many craters and small mountains. Venus also has a rocky surface with impact craters, mountains, lava plains, and volcanoes. Earth, as we know, has large landmasses with valleys, mountains, volcanoes, and plains, along with large oceans of water. Mars is currently void of liquid water, though evidence suggests it was once prominent on Mars' surface due to erosion features. Most of the water is now trapped in icecaps or possibly below the surface. It's also home to two large volcanic systems, much bigger than the ones we have here on Earth.
Jupiter's surface is covered in thick red, brown, yellow, and white clouds, and thunderstorms much larger than the ones here rage there; not only that, but large winds occur there, too. Saturn is known for its large rings that orbit its equator, and, like Jupiter, doesn't exactly have a solid surface, as it's a gas giant. The gases in the atmosphere create the bands of color we see in pictures of these planets. Uranus, too, lacks a solid surface, and is mostly rock and ice, though that's more the interior of the planet. Neptune, being a gas giant like the other three, is also without a solid surface, and is also made mostly of rock and ice, and its blue color is caused by a large absorption of red light by methane in the atmosphere. It has the fastest winds of any planet in the solar system, too, with speeds up to 1,000 km/hr. Other planets outside of the solar system currently fit in to the terrain/gas giant classification, though their terrains vary based on their composition. Gas giants like Jupiter, Saturn, Uranus, and Neptune don't have a solid surface, and the surfaces of the terrestrial planets vary just like they do here, depending on the most prominent element. One of the most interesting planets discovered actually has a composition that is mostly diamond. In a binary star system, one star exploded into a supernova, while the other became a white dwarf. The one that exploded became a pulsar with a strong gravitational pull, taking most of the matter from the white dwarf until only its carbon core was left. And once carbon is heated, it becomes diamond; so, the core was heated, and ta-da, a diamond planet was born. So, the terrain of planets varies based on their composition.