Halftone Separation Film is produced when your digital image files and page layout files are Raster Image Processed (RIP’ed) and sent to an image setter. In this process your low-resolution FPO (For Position Only) image files are swapped out for your high-resolution image files. In the RIP your images, graphics, and typography are combined and output to an imagesetter. It is at this point that the halftone screens are applied to your images. The imagesetter produces four pieces of Halftone negative film (black and clear only, with no intermediate shades of gray). One film each for the Cyan, Magenta, Yellow, and Black Printing Plates. The film (or separations) are not in color themselves. They are simply the film that images the information separately for each of the process colors.
Direct to Plate: Often refereed to as Computer-to-Plate (CPT). In this process the files are RIP’ed and output on a device that produces an offset printing plate directly from the file without having to make halftone negative film. Because there is no halftone negative film you cannot make Matchprints and must rely on digital proofs such as an Iris Print or a Kodak Digital Contract Proof. Again, the plates (or separations) are not in color themselves. They are simply the plate that images the information separately for each of the process colors. (They actually could be inked up in any color you wanted.)
MatchPrints: Matchprint II, PressMatch, and Cromalins are contract proofs which are made directly from same halftone film that will be used to make the actual printing plates. Because they are made from the actual halftone separation film and because the processes are specifically calibrated to match process ink on paper they can be used as contract proofs, that is to say the printer agrees that he can achieve with ink on paper what you see on the contract proof.
Plate Making: In the platemaking process the halftone negative films are assembled into blocks of pages called signatures that fit the size of the particular printing press. At this point the pages are Imposed. Imposition is the process of adjusting the page layout to account for the varying amount of image lost in the gutter when the book is bound. Also at this point that the platemaker adjusts the images to accommodate the dot gain of the particular press, ink, and paper combination.