We are always PMSing here at 22. PMS, as in "Pantone Matching System," that is. Pantone (PMS) is a color matching system commonly used in the design and printing industry with a very practical use...
Here's the problem Pantone solves: if you try to describe a color you have in mind using terms such as “robin's egg blue” or “tweety-bird yellow," it becomes quite tricky as you will envision one color and the designer will likely envision another. Furthermore, since all computer monitors are calibrated differently, you can't rely on what you see on your screen to be the same as what a designer or printer will see on his screen, either. These conditions can lead to a lot of unnecessary back-and-forth or misunderstanding over the simple matter of choosing a color.

You can end this miscommunication with the use of PMS swatches and everyone will be speaking the same language: you choose the color swatches you like and you know that the result will match the swatches you've chosen. PMS swatches are just like the paint swatches you get in the local hardware store, but instead of mixing up paints to specification, we're mixing up inks.

These PMS color swatches have exact CMYK break-downs to create your desired color (CMYK stands for Cyan, Magenta, Yellow and blacK, the 4-colors used in full-color printing). No longer is there a guessing game; choose the PMS color you like and the printer will match it. Keep in mind, however, that if you want a very exact, precise color, this PMS choice must be designated as a "spot color." If not designated specifically as a spot color, the 4-color breakdown of the PMS color will come very close, but will not be as precise as when a spot color has been specified. Spot colors are, of course, slightly more expensive.
Additionally, there are special inks, such as fluorescent or metallics, which must be spot colors. There is also the option to "double-hit" a spot color (print twice) for a stronger, more opaque color. All of this can be specified with PMS swatches. PMS swatches also show how different color breakdowns will appear on different stocks of paper, such as glossy or matte finish.
Find out more about PMS at: http://www.pantone.com/pages/pantone/index.aspx