Press Plate: Also called "printing plates" or "plate cards." Almost all cards are made from pressing ink-coated metal plates against blank cardstock. Most printing involves the four "CMYK" colors: cyan, magenta, yellow and black. The plates all show the same photo but have different etchings depending on the color of ink to be applied. For instance, the yellow press plate for a 1970s Pittsburgh Pirates player will be heavily etched, while the cyan plate will barely have any etching, due to the heavy use of yellow in Pirates uniforms of that era. In the mid-80's Topps put up for public auction many of their old plates, an unprecedented move. These plates always command interest, with top stars bringing great prices.
In 1997, Pinnacle started inserting the press plates used to make the set into packs, starting with the 1997 New Pinnacle brand. Actually, the plates found in packs aren't really the plates used, but some sort of color test plate. If they were the real plates, the images would be the reverse of what you see on the card.
Since then, plates have appeared in Stadium Club, Pinnacle, and Topps Gallery. While practically unique, each plate comes in four variations, so they generally aren't considered one-of-ones. Value varies greatly, depending on player, whether the plate is for the front or back, and the clarity of the etching. Many player's plates are unidentifiable because they are so faint (a problem most common with the yellows). Even comparing them to the regular cards sometimes can leave the player in doubt. Prices range from around $20 for a common back plate to around $1000 for a clearly-defined front plate of an elite player.