Warcraft Encyclopedia: The Warcraft Universe

To date, three planets have been specified in the Warcraft universe: Argus, Azeroth and Outland. However, countless other planets exist in the Great Dark Beyond.

Long before recorded history, the benevolent titans came to Azeroth and instilled order where there was chaos. In the process, they banished the planet's destructive elementals to the Elemental Plane, an abyssal plane specially created by the titans. To this day the vast majority of elementals continue to inhabit the Elemental Plane, where they are constantly at war with one another.

The Twisting Nether is an astral plane that overlaps with the Great Dark Beyond, but is not normally perceptible to mortals.

References

The information could still retrieved from http://www.wow-europe.com/en/info/encyclopedia/580.xml.