Realities

Realities are various different results of events rippling across time, with one reality's events occurring separate from others. For example, in one reality, Bruce Wayne became Batman when his parents were killed, but in another, it was Bruce who was killed and as a result his father Thomas became Batman. The difference is that the vibrational string frequency that governs all matter and energy is universally different across realities. In the Bruce as Batman universe the strings vibrate at one uniform (to a very close decimal) frequency and in the Thomas as Batman universe the strings vibrate at a different uniform frequency. The closeness of these frequencies from universe to universe governs how similar events across time are, or at what point they diverge.
In terms of simple orders of magnitude, the vibrational difference from reality to reality is the largest, with the difference from universe to universe being the smallest.
Overall, almost all realities belong to a Multireality, which is grouped by vast similarity in frequency. One Multireality typically exhibits the existence of the same individuals across all or most singular realities, whereas another multireality has entirely different circumstance and characters and even laws of physics. All Multirealities are contained within the Omnireality, which includes every conceivable existence excluding Heaven, Hell, Dreamworld, Limbo, and Purgatory-- dodectillions of possibilities. 
Below Realities in terms of vibration magnitude are timelines. A timeline is the sequence of events from the beginning of that reality until its very end, and can be disrupted by time travel. Similarly to the difference in realities, a timeline difference is defined by minuscule or massive changes in sequence of historical events. If the bullet that hit Thomas Wayne didn't kill him because of some alteration by a time traveler, this would spawn a new timeline where Thomas Wayne was not killed by Joe Chill. However, unlike realities, Timelines cannot be affected by arbitrary changes that have seemingly no context; all changes from timeline to timeline are based on the previous sequence of events and minute changes within.
After Timelines are Upside Downs. For each reality there is only one upside down, but it exists accordingly differently across all timelines. The Upside Down of the Primary Reality, referred to as Bizarro World, is vastly similar, but the personalities and actions of individuals and anything relating to or created by those individuals has been reversed-- the good are evil and the brilliant are inept. Upside Downs occupy the same space as "Rightside Ups" and thus, landmarks like Wayne Tower exist as they would on the other side. 
Next down are dimensions. Dimensions are far enough vibrationally that they do not occupy the same "side" of space, if space is considered akin to a cube, but close enough that chronological events and time-based entities (such as life forms) do not appear in different form from dimension to dimension. Typically, dimensions are connected by energies and have vastly different laws of physics and lifeforms.
Below dimensions are Realms, which are close enough in terms of string vibration that they exist in the same sense of space and thus follow the same physics, but far enough that one cannot be accessed by another by simple space travel. Though realms do play by the same rules of physics, the beings within different realms often have different control over said rules, with beings like Olympians being able to alter the rules to some extent but typical beings on Midworld not having that ability. The most important realms are Midworld, home to mankind, Olympus, home to the "Greek" gods, Apokolips and New Genisis, home to the New gods, and Asgard, former home to the "Norse" gods. 
The most significant early barrier in string travel is that of space. The highest magnitude by which one can still travel via conventional means is Universes. Universes exist close enough that theoretically, one could travel through outer space in a space ship from one to another, but the distance is so extreme that this is virtually impossible for mortals, and alternate methods are necessitated.
At the lowest level, within one universe, is close physical space as it is known to the layperson. All observed by common scientists, galaxies, stars, planets, etcetera, exist within one universe and could hypothetically be visited by conventional space travel.
Travel across the levels of reality is typically a form of teleportation; a place where magic and science meet. By altering the vibration of an individual's strings, they can be teleported to the alternate reality or dimensions without necessarily moving through physical space. The larger the difference in string frequency, the further the transportation achieved.