A plotter is a graphics printer that literally uses ink pens to draw the images. The pens move around on the surface of the paper like something out of The Sorcerer's Apprentice. Plotters can only draw data in vector graphics format, graphics that are made of straight lines (the curved forms are actually drawn with many tiny straight lines). There are flatbed plotters where the pen moves across the page in the x and y axes.
There are drum plotters where the drum moves along one axis while the pen moves along the other axis and the paper rolls through. There are pinch-roller plotters where the pen moves along one axis and the paper moves back and forth on small rollers. Some plotters draw the image with electro statically charged dots, then apply the toner and fuse it, similar to laser printers. Plotters are very interesting and often fun to watch, especially the fast ones.