The "Stepper Box" (or just "Stepper") has a three way switch on top of it, one marked “WEST”, one "OFF" and the last “EAST". “West" and “East" are the names given to the two directions of travel possible through the Long Earth - flicking the switch to either causes the user to be transported one world in that direction.
In all of the Long Earth, humans only evolved on one version of the planet, known as "Datum Earth", or "the Datum".
Worlds that don’t take many ‘steps’ to be reached from Datum Earth are very similar to it (apart from the absence of Homo sapiens). The further you travel through the Long Earth away from the Datum, the larger and more noticeable the differences become.
In the Long Earth, worlds are named after the amount of steps - either ‘West’ or ‘East’ - that need to be taken to reach it from the Datum. For example, 'Earth West 5' is located five steps West of Datum Earth.
It is unknown whether the Long Earth is infinite or not, but so far no end of it has been discovered.