The Great Dark is similar to Outer Space.
The Great Dark, or Great Dark Beyond, is the empty void between worlds. It is described as the first of everything when there was nothing, and is part of the primary material dimension, as opposed to the Twisting Nether. It is essentially the name given to outer space in 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. [1]
The Titans probably travel through the Great Dark, as opposed to the Twisting Nether (which might explain their shock at the emergence of the Demons).
Great Dark vs. Twisting Nether
The Twisting Nether is the astral plane between worlds. It is a chaotic and magical environment that overlaps with the Great Dark Beyond, yet is normally imperceptible to mortals. [2]
The roles of both have not been clearly confirmed yet. For example, in the novel Last Guardian, Medivh explained to Khadgar that demons were denizens and rulers of the Great Dark, and referred to the Dark Portal as a gateway that bridges two nodes residing in Twisting Nether. Later WarCraft material constantly referred to demons dwelling in the Twisting Nether. However, if the information Medivh gave the young Khadgar is to be considered canon, then the Twisting Nether could be an alternate dimension or a "hyperspace" of sorts, where the conventional laws of physics, time and space do not apply and thus allows for instantaneous intergalactic travel.
Aegwynn, in her narrative at the start of the Second War, referred to the Great Dark in much the way later sources established the Twisting Nether, so we can assume it was an error or a retcon. The usage of Nether and Great Dark was more ambiguous in Warcraft II than in later games. Though Aegwynn claimed to be responsible for defending Azeroth from forces in the Great Dark, later sources clearly established that her duties dealt with the Nether.
It is possible that Aegwynn chose to use Great Dark instead of specifying the Twisting Nether because her duties extended to defending Azeroth against harmful entities that are not solely restricted to the Twisting Nether. Indeed, it is chillingly possible that there are forces in the Great Dark to rival the Burning Legion yet unknown to the powers that vie for Azeroth.
Warcraft RPG implies that Twisting Nether is a smaller part of the Great Dark, but that there is no specific definable boundary between the two. This was likely made to explain why the terms are sometimes interchangeable. The Warcraft Encyclopedia now confirms that the Great Dark includes the Twisting Nether, rather then vice-versa.