Multiverse

The Multiverse, sometimes called the "Omniverse", is the infinite number of universes that are layered atop of each other. Each universe may be completely different from another, whilst some are merely reflections of each other with ever so slight differences. Very few have harnessed the ability to deliberately cross the borders blocking access between universes, however it is known that universal mechanics often result in unpredictability and freak phenomenon that can open tears - sometimes permenant - that people have crossed, sometimes by mistake.

Functionality of the Multiverse
A parrallel universe are universes that run alongside each other (parrallel) and are usually very similar, possibly containing the same people, locations and physics. If one were to cross into a parrallel universe they may find it is identical, save perhaps a leaf blew in a different direction one day. On the other hand they may find it very different than their own where history has taken a completely different turn.
 * Parrallel Universes

The most commonly exploited incident of parrallel universes is Earth and humans. Although not always from parrallel universes, humans are the most prevalent sentient species in the Multiverse and often they originate from the planet Earth -- yet these Earths and humans are often not from the same universe.

Usually parrallel universes are much sturdier than alternate universes as their borders are much closer, making it much less likely for someone to encounter themselves from a parrallel world. However humans and Earth often break through these expected barriers. Some believe that universes are being created through echoes in the Multiverse, an image of an incident in one universe echoes outward and produces additional universes suddenly and with weakened "walls of reality".

Alternate universes are universes so far removed from all others that they have little to no resemblence to other universes. Each alternate universe would have its own parrallel universes.
 * Alternate Universes


 * The Void


 * Portals


 * Echoes
 * Walls of Reality

External Sources

 * " The multiverse  (or meta-universe ) is the hypothetical  set of multiple possible universes  (including the historical universe we consistently experience) that together comprise everything that exists and can exist: the entirety of space, time, matter, and energy  as well as the physical laws  and constants   that describe them." - Wikipedia opening statement.


 * "A specific group of parallel universes is called a "multiverse", although this term can also be used to describe the possible parallel universes that constitute reality. While the terms "parallel universe" and "alternative reality" are generally synonomous a nd can be used interchangeably in most cases, there is sometimes an additional connotation implied with the term "alternative reality" that implies that the reality is a variant of our own. The term "parallel universe" is more general, without any connotations implying a relationship, or lack of relationship, with our own universe." - Wikipedia's explanation.


 * "     Within Marvel Comics, most tales take place within the fictional Marvel Universe, which in turn is part of a larger multiverse    . [...] Each universe has a Captain Britain designated to protect its version of the British Isles." - The Free Dictionary gives an example of the multiverse being used by Marvel Comics.