Body refers to bio-chemistry and organics, defining what is possible for organic life in a given world. Technology refers primarily to traditional physics and the behavior of equipment and machines. Psionics refers to psychic phenomenon (from basic ESP to physically impossible acts of mind over matter). Magic covers not just spells, but the supernatural world at large, using the explanation that the ritual of a spell (a spell being anything from a prayer to alchemy) channels expectation of the spell-caster into a desired effect. Along with this explanation comes the system's supernatural order that divides magic into four categories: Arcane Magic, Alliance Holy Magic, Anarch Holy Magic, and Neutral Holy Magic. These categories discern whether you are calling on a deity to act through you (Alliance=Good, Anarch=Bad, and Neutral is self-explanatory) or whether you are drawing on raw supernatural energy yourself (Arcane).
Multiverser is not a generic game like Hero System or GURPS, or cross-genre game like Rifts, but a "campaign system", with a discrete back story. The best comparison is the television series Sliders, but players are not limited to "mundane" alternate earths, with the same rules of reality as our own. "In the Multiverse, every story is true somewhere. There is no fiction." Everything is true in some universe.
The "plot immunity for heroes" problem drains excitement from action-adventure TV shows, and role-playing games. Multiverser retains the fear without game-ending consequences by letting a player character be killed, and then transferred to another material world to start another adventure. However, this means that the player was defeated and exiled from the first world.
This is ahcieved with a substance called scriff, a golden colored mercury-like substance that behaves like a liquid, but is not matter. It is an ether of sorts, binding the universes together and transporting the players. Players usually become infected with scriff when they suffer a deadly accident caused by a "scriff enhanced" appliance, though it isn't the only way to travel the Multiverse.