Existence, also called Mundus, is the broadest thing in the universes. Best put, Existence is the total sum of everything, everywhere, that exists. It encompasses the four realms that exist; Aetherius (the oldest of them all, home realm of the Gods), Infernus (the second oldest, the small realm of Satan, and his daemon servants), Atmah (the third oldest, created and sustained by the Divines. Includes Earth, the Milky Way, and the Andromeda Galaxy), and finally, Lenor (the youngest universe, home of Artem, Equestria and the ponies). There once existed a 5th universe, known as Klendathu, created by Sargeras in cohesion with the Dark Gods.
Mundus is what has become known as a "completed" form of Existence - that is, it is the realization of Aetheria and Anui's goal, an ordered form of Creation (see the Creation Story). This is converse to Araboth, the only other form of Existence, created by Anui and Sargeras, which was not completed, due to Aetheria's intervention.
The opposite of Existence is Nothingness. This is also more commonly referred to as the Void. The Void is a dark, ever-shifting realm of infinite darkness and chaos, where nothing remains static or orderly. This is the domain of the most ancient evil of all, the origin of all chaos and darkness, Y'Shaarj.