Planets are celestial bodies that almost always have an orbit around a sun (those few which don't are referred to as "rogue planets"). They can be any distance from a sun, from almost touching the corona of a sun, to being so far away from the star that they orbit that the sun is just like another small speck in the sky.
Habitable planets, such as ones that support advanced civilizations such as the Vedrans and various other species, often lie in a temperate zone, which is band a certain distance away from the sun that makes conditions for life ideal. Planets can also have different diameters and atmospheric conditions, from oxygen atmospheres with tepid temperatures to frozen ones that are inhospitable for any kind of life.
Planets also have many different forms, such as gas giants that are hundreds of times larger than smaller, rocky planets. There are also planets that are not much bigger than asteroids, but meet varying criteria for being a planet.
Moons orbit planets which in turn orbit suns.