The Warcraft Multiverse was created through the contesting of the Light and Void. The clashing of these two opposite forces spun innumerable celestial bodies into existence and created wonderous life forms on them.
There are an infinite number of timelines in the WoW multiverse.
Places of Interest in the Warcraft Multiverse
The Void(Also called The Shadow) is a void of nothingness outside of creation. It aided in the creation of the multiverse by clashing with the Light. It is described as boundless, endless, nightmarish, and eternal. It is semi-sentient and seeks to absorb all matter and energy to satisfy it's infinite hunger. There are beings called Void Lords who are the supreme rulers of the void. They are envious of the Titan's power and created the Old Gods to infect a Titan soul to herald their arrival into creation. 
The Twisting Nether
The Twisting Nether(Also called the Nether) is a chaotic astral dimension separated from and connected to all realities. It is incomprehensible to mortal minds, time flows differently, and laws of physics are warped. It is where Demons go to reform when they are destroyed. It was created when the Light and Void clashed, it represents the chaos between the two forces. 
The Shadowlands are a labyrinthine alternate plane of existence where the dead souls of the universe can end up. It is described as a dark gloomy place where soul eaters torture the lost spirits of those who are unfortunate enough to enter it.
The Caverns of Time
The Caverns of Time is a place where any point in time is accessible. The Bronze Dragonflight were given the task of protecting it and keeping history on it's natural course. And to make sure no alternate Azeroth's exist.
Titans - Planet-Sized celestial beings made of pure Arcane Magic.
Void Lords - The beings that embody the unholy void.
Old Gods - The servants of the Void Lords and physical manifestations of the Void.
Naaru - Cosmic benevolent entities made of holy light.
Demons - Chaotic entities made of Fel Magic. They all transcend reality so there is only one of them across all of them.
Loa - Powerful spirits that gain power from worship.
- Demons and those infused heavily with Fel Magic transcend all realities.
- The Titans are similar to Marvel's Celestials.