The role of the structure is to provide a rigid platform for the machines components to be secured to. The material of the structure can be as unique as the machine itself. Some common materials are plywood, acrylic, and metal.
The XY axis (E):
The XY axis provides stable, linear movement in the X & Y directions. This mechanism is controlled by either a belt or screw interface to a stepper or servo motor. The amount of XY movement per step correlates to the machines resolution.
The Z axis (A):
The Z axis elevates the extruder above the build platform and moves perpendicular to the XY axis. This mechanism is typically controlled by a linear screw interface to a stepper or servo motor. The amount of +Z movement correlates to the thickness of each layer. Some machines move the extruder with the XY axis and the Z axis moves the build platform. Regardless of the configuration, all three axis are required to construct a 3D object.
The Build Platform (D):
The build platform is the surface that supports the extruded material while the machine completes each layer. There are two types of build platforms, heated and not heated. A heated platform is optimal for larger parts as it helps to eliminate the problem of thermal contraction of the extruded material which leads to part deformation and warping. Printers that print ABS often require a heated platform while those that use PLA do not.
The Extruder (C):
The extruder is the heart of the 3D printer. This mechanism is responsible for accurately depositing the desired feed stock onto the build platform. There are numerous designs and types of materials the extruder can handle. If it is extruding plastic feed stock, the extruder relies on a toothed feed wheel that feeds the plastic filament into a heated chamber. This chamber is thermally regulated to the desired melting point of the plastic and once molten, the filament is forced through a small nozzle. The diameter of the nozzle dictates the detail that can be replicated by the machine and are generally ~0.4mm - 0.5mm in diameter.
The Electronics & Software (F, G):
The electronics and software make up the brain of the 3D printer. Designs are produced on a computer as a solid model and are deconstructed by a G-Code processor (Skeinforge, Slicr, Miracle Grue, etc.). This piece of software takes the solid model and "slices" it into a series of layers. Each layer is then converted into a series of coordinate movements that are fed into the machines controller by the control software (ReplicatorG, MakerWare, Repeiter-Host, Pronterface, etc.).