A planet is a spherical celestial body which orbits a star and is massive enough to sustain an atmosphere (which it may or may not actually have), but not so massive that ignites into a star. They can be gaseous, such as Saturn and Jupiter, or solid like Earth and Mars.
Planets are a mystery wrapped in a riddle so-to-speak, no one knows how most are formed. There are 3 basic classes of Planet out there, 3 of which we know about it, 1 that technically doesn't count.
The first are the Terrestrial Class, these are the solid planet's that we can colonize today, while there are few in this solar system that we can colonize effectively, they are the only celestial bodies in this galaxy that we can we can use to either build settlement's are mine for resources. Terrestrial planets come in all sorts of shapes and sizes, but many of which hold basic characteristic's, some have no atmosphere to speak of, this is due to that fact that the atmosphere might have been blown off or had no atmosphere to begin with, making the colonization of that one planet very hard to pull off, this can be effectively countered using terraforming method's, but with the economic issues of today, this seems highly unlikely. The Temperature of most planets might be considered a problem, mainly because we can only sustain a certain temperature before we burn to death, planet's that have these problem's are Venus due to it's thick CO2 atmosphere which creates a greenhouse effect, and Mercury due to its high Velocity orbit and distance from the sun.
The second are Gas Giants; these are the planets that have no land what-so-ever, these planets that have gas to build their atmosphere, making them incredibly large. With a huge and dense atmosphere, it's hard to figure out what can be found in the middle, most scientist believe that a core of rock lies in the middle, this has yet to be seen. Most Gas Giant's are believed to have huge deposit's of Helium-3, an important source of energy for out future Nuclear based economy, making Gas Giants a highly sought out target for mining divisions in the near future, but you wonder how do we mine them if they have no land to land on, well this can be achieved using Space Cities, or moon's to sustain both a population, and a mining base.
Dwarf Planets are smaller Terrestrial Planets, even smaller than our own moon, these objects are perhaps the most mysterious out of all the planets in our solar system, mainly because we haven't clearly seen one at the moment, the two most mysterious objects in our solar system ought to be Pluto an Eris, two which planets we have never seen. It is very hard to spot Pluto and Eris because they have a very odd way of orbiting, mainly because Pluto intersects Neptune's orbit, and Eris has a very eccentric way of orbiting, because it travels on a north and south axis of the solar system.
Moons can't be considered terrestrial planets, but they both draw similarities to each other, mainly because they are both made of solid dense material's, and for some moon's out there, they do contain an atmosphere (See Titan for more details). They can't be considered planet's mainly because they orbit there owning planet, these moon's can be considered to be very important, mainly because some can be colonized for a very big population to live on, on Gas Giant's they are particularly important to sustain Mining Families, and factories to process the resources.
Planets are featured in a vast number of games, particularly ones with a science fiction theme where travel between different worlds has become possible. In adventure games like Metroid Prime 3 or RPGs like Rogue Galaxy, characters may visit different planets in the course of their quest, while strategy games like Galactic Civilizations usually focus more on colonizing and conquering many different worlds.