Planets are large celestial bodies that, with the decidedly arbitrary exception of a variety called gameworlds, each orbit a star or stellar remnant known as a sun and are sometimes orbited by one or more natural satellites known as moons. They are usually rounded into a spherical shape by their own gravity but on rare occasions can be shaped like a cube or other form. Many planets are inhabited by living organisms ranging from mindless bacteria to complex sapient beings. Those that are not are usually colonized and exploited at some point by said sapient beings.
Individual planets are sometimes referred to with the word "planet" as a prefix, i.e. "planet Miblok" instead of just "Miblok". This is of course grammatically unnecessary but helps to differentiate them from asteroids, comets, moons, or whatever else someone may be referring to, especially seeing as there are too many planet names for anyone to keep track of. There are a few exceptions such as Planet X which clearly have the word "planet" in the name, and that saves a lot of bother and complication all around.
The prospect of destroying a planet was once technologically and morally unthinkable to most beings, aside from Galactus, who devoured them. However, technological and amoral developments have made it childs' play in many circles. Massive space stations known as the Death Star and Death Star II were among the first superweapons developed for this purpose, although it was claimed but never demonstrated that the earlier and somewhat smaller satellite Trigon One could do it as well.
Elsewhere and much later, similar technology evolved to the point where it could be spread out collectively amongst the ships of a Vogon Constructor Fleet, and planets could be routinely demolished for development purposes. At one point a planet in the Seventh Dimension was reportedly used as a ball in a game of intergalactic bar billiards and potted into a black hole. Tragically, it only scored thirty points.
The technology of building planets is more complex still and was developed by the Magratheans of Magrathea in response to dissatisfaction from some very rich people with the worlds they had settled on. It involves sucking matter through white holes in space and assembling it within a massive "factory floor" space.