The Universal World Tree
The Universal World Tree is the collection of every multiverse created by Wabbajack.
This world started off as a singularity; nothing. Not even time and space existed. This time is known as the Origin. Entropy was at it's maximum. Everything changed when a sudden decrease in entropy caused the spontaneous creation of a Universe.
It's Main Timeline, consisting of the universe we know and love.
Let's not get ahead of ourselves. At the same moment the Main Timeline was being combusted, a seemingly infinite number of timelines formed as well.
These timelines are vastly different from our own due to small changes in how entropy attempted to return to maximum. The smallest difference in energy placement resulted in completely unique ways time, space, and physics performed. Some universes experience slower universal time than others. One second in one universe might by 87,456 years in another.
The universes used in different campaigns are very similar to the Main Timeline. They often take place in Earth or an Earth-like planet with carbon-based life. The contents of these worlds vary as well, often in how biological life evolved. What we describe as monsters appear in many of the different worlds.
Of course, everything can be completely different as well. For all that we know, our PC's could be piloting spacecraft as silicon-based tentacle monsters. While that could be fun, I find PC's like to play more huminoid characters.
Let the games begin.