First, the platform itself heats (to a default 110 degrees Celsius), followed by the extruders (220 degrees Celsius by default).

ENGADGET: MakerBot Replicator impressions: the dawning of 3D printers in every home?

While not always immediately apparent how, you can easily make alterations to the object, including alternating nozzles to switch up its color for dual extruders.

ENGADGET: MakerBot Replicator impressions: the dawning of 3D printers in every home?

The heads operate along the X and Y axes, while the build platform (generally heated in the case of ABS and unheated for PLA) moves downward, allowing the glue gun-like extruders to build up the thin layers of plastic.

ENGADGET

The extruders, naturally, melt the plastic down, while the heat from the platform plays a role in keeping the object in place as it's printed -- or, more precisely, the lattice work foundation that object is built on top of.

ENGADGET: MakerBot Replicator impressions: the dawning of 3D printers in every home?