Once the schematics for the image are complete, the computer downloads the coordinates to the plotter, which interprets the code and calculates the most efficient path for the pen and paper. (eHow, Inc.)

Pen plotters work by moving a pen across the surface of a piece of paper, meaning that plotters are confined to line art, and are incapable of producing rasterized graphics like other printers. It is possible to draw complex line art designs and text, but pen plotters are considerably slower, owing to the pen’s mechanical movements. (richcolorprinters.com)

Plotters are used primarily in technical drawing and CAD applications, where they have the advantage of working on very large paper sizes while maintaining high resolution. (reference.com)