With the first model plotters, the plotter basically used a pen nib to draw maps ect, using co ordinate based software and generally printed on rolls of paper.
Where as a printer, prints on sheets of paper and geneally prints graphical or text based documents. however nowadays, plotters, can be widely referred to as large format.
Source: http://wiki.answers.com/Q/What_is_the_difference_between_a_printer_a_plotter