"The Bus" is the common name of the school bus that Ms. Frizzle's class and Liz ride on and goes on field trips. The bus is most often piloted by Ms. Frizzle or Liz, but occasionally, one of the kids will take the wheel (i.e: Ralphie in Kicks up a Storm and Carlos inGets Ready, Set, Dough) or the bus will take on a shape (such as an animal) that allows it to "drive" itself.
It isn't really clear how the Bus works, but there are some hints in the TV series. We can see the how the fuel get to the engine in the episodeRevving up. First of all, its magic seems to flow from several unusually named devices, like the shrinkerscope or the mesmerglober. The shrinkerscope serves mostly as an easy way to shrink and resize The Bus at the will of Ms. Frizzle. The Bus seems to have at least two varieties of the shrinkerscope, namely, the main one (as an internal part of the Bus) and the porta-shrinker. At least one of these two devices (porta-shrinker) can't work when damp or wet; and if someone attempts to activate it in such conditions, the dew-dinger sounds.
The mesmerglober, on the other hand, is primarily concerned with changing the shape of the bus. It changes the Bus into whatever the class needs, be it a submarine, a spaceship, or a bus-igator.
Images of Bus Transformations