Planes are normally completely flat, aside from any mountains or valley they might have, as opposed to planets, which are round and orbit stars. Planes do not need to follow any normal laws of physics, and may have no sun or sky, no borders, no gravity, no surface, or other strange features impossible in the Prime Realm.
Planes are remnants of Vel'kira, a much more massive plane which destabilized and broke apart in a long ago age. As such, many are still inhabited by the races that once inhabited Vel'kira, having a common origin, despite having had no direct connection to one another in a very long time. Thus they may have developed completely different cultures.