Many fans wonder what the Twisting Nether and the Great Dark Beyond are. A fan offered a very good example of what they are:
“You have the universe, the universe is made up of two plane of existance.
The solid plain of existance, the one in which the suns, planets, and moons form, is known as “The Great Dark Beyond”. This is basically the name that the people of Azeroth use as a word for “outerspace”. This physical realm is where the planet of Azeroth sits, and is the realm in which the Titans travel the cosmos searching for worlds to re-construct into order.
The second plane of existance is an ethreal plane, one made of protein that has no form, no planets, nothing but an empty infinite expanse. This is called the “Twisted Nether”, and is considered the “glue” of the universe, as the entire nether is woven into the rest of the physical universe. The Titans know of the Twisted Nether, but because it is such a chaotic world, they refuse to travel within it. The chaotic nature of the twisted nether was a perfect home for those that gave themselves to chaos, the races that would later become known as demons.
The relationship between these two dimensions is to hold the universe together.
One of the perks the demons gain from the Twisted Nether, is the fact that the Twisted Nether, is agian, the glue of the universe, this means that every single point of the twisted nether, is also connected to every single point of the great dark beyond. One could enter the twisted nether by portal, walk three steps through another portal, and be on the other side of the universe.
The Twisted Nether was used is this way to bring the orcs.
Medivh used the twisted nether as a “bridge” across the universe, allowing the orcs to move instantly from their homeworld of Draenor, all the way to Azeroth within an instant” - Grocalis @ Tichondrius
A very detailed and descriptive explanation of the Twisting Nether—more official, from Chris Metzen—may be read at Warcraft RPG: Shadow and Light page 152-160