When you take a photograph on your DSLR, your camera will compose your shot using an RGB spectrum. RGB stands for red, green, and blue and refers to the three primary colors that can always be mixed together in different variants to produce different colors.
Computer monitors also work in RGB, which is why DSLRs follow this format to make things easier for users. RGB is known as an additive color spectrum, as it relies on adding different amounts of three colors to make different colors. In the RGB spectrum, there are 256 levels of brightness which produce 16,777,216 (256x256x256) color possibilities. By setting each RGB color to a setting of "0," we can generate black, while a setting of "255" generates white.
Therefore, RGB is the industry default for DSLRs and computer monitors, as it allows us to view colors true to life on screen.
However, if we want to print our images using a correct color spectrum, we need to convert to CMYK. This stands for cyan, magenta, yellow, and black. CMYK is a subtractive color spectrum, as cyan, magenta, and yellow pigments are used as filters. This means that they subtract various amounts of red, green, and blue from white light to produce different colors.
Therefore, an image displayed on a computer monitor may not match a print, unless the RGB spectrum is converted to CMYK. Although many printers now convert from RGB to CMYK automatically, the process is not yet perfect. As RGB doesn't have a dedicated black channel, blacks can often appear too rich.
If you take your work to a dedicated printer, they will ask for the image in CMYK, as it's the format in which they will have always worked. CMYK dates back to the days of color printing and processing. However, it seems likely that RGB will eventually become the industry norm, as more people move away from traditional methods, and digital becomes more and more prevalent.