ReplicatorG is a program that runs on your computer. It's interface is similar to Processing. It translates the GCode into Sanguino3G, which the firmware understands. The firmware runs on many Atmel AVRs (like the Arduino compatible Sanguino), which control many varieties of 3D printers and CNC machines. The firmware generates motor step and direction pulses which are sent to the motor controller.
Right now the firmware / replicatorg supports 3 modes: print over serial, upload to SD over serial, and print to file (such as an SD card). it also lets you start a build from a preformatted file on the sd card.
The firmware only supports Trivial Kinematics: one motor mapped 1:1 to each Cartesian axis.