text: The plates are the actual metal sheets onto which the text or pictures are transfered.  Printing ink is applied to them, it only sticks to the design, and then paper is pressed to the plate, transfering the design to the paper.  Usually, only one plate is used per color, which is why its cheaper to print in just black and white with maybe a "spot color" (when there is only one specific color other than black).  When you print something in full color (like a photo), you have to use four plates (black, magenta, cyan -- a blue-like color, and yellow) to make up all the colors.  These days, many printers are using digital printing, no plates required, but for mass print jobs, plates are still cheaper.\n\nIn high-end printing (like art books), "plates" sometimes refer to the full-color pages needed to reproduce paintings or photos.  The plates are different paper and printing process than the rest of the book to avoid having to print the entire book in full color.  This terminology is left over from when books were printed letter by letter, which left no room for illustration.  Contrary to what the first poster states, the plates were etchings that had to be hand colored, had to be printed seperately from the words and added to the book when it was assembled.