CMYK stands for Cyan, Magenta, Yellow, and black. The black is referred to as K denoting key, a shorthand for the printing term key plate.
These are the four ink colors most commercial printing presses use to create all the other colors. If you have a “full color” image, your printer will print with CMYK inks to reproduce the image.
RGB stands for Red, Green, and Blue. These are the 3 colors that are used in electronic media (like TV’s and computer monitors) to create all the other colors. If you supply your print files with colors created from RGB, your print shop will have to convert these files to CMYK. Sounds simple right? And it can be, but other times it can create problems (not to mention an additional charge to convert your files). Converting files from RGB to CMYK can cause the colors to shift in an undesired manner. It can even cause your images to get fuzzy or pixelated.
If all you have are RGB images, it is best for you to convert them yourself (this is easily done in Photoshop with just a click of a button). This will not only save you money on prepress / file charges, but you will also be able to see what the final converted image looks like and can manipulate it further if needed.
4/4 pronounced “four over four”; Four ink colors applied to the front of the piece and four ink colors applied to the back – usually the 4 colors are CMYK
4/1 pronounced “four over one”; One side of the printed piece has the four ink colors applied to it but the other side only receives one ink color
4/0 pronounced “four over zero”; One side of the printed piece has the four color ink applied to it and the other side has no ink (blank)