What is Pantone (pms)?
A Color Matching System, or CMS, is a method used to ensure that colors remain as consistent as possible, regardless of the device/medium displaying the color. Keeping color from varying across mediums is very difficult because not only is color subjective to some extent, but also because devices use a wide range of technologies to display color.
There are many different color matching systems availlable today, but by far, the most popular in the printing industry is the Pantone Matching System, or PMS. PMS is a "solid-color" matching system, used primarily for specifying second or third colors in printing, meaning colors in addition to black, (although, obviously, one can certainly print a one-color piece using a PMS color and no black all).
Many printers keep an array of base Pantone inks in their shops, such as Warm Red, Rubine Red, Green, Yellow, Reflex Blue, and Violet. Most PMS colors have a "recipe" that the printer follows to create the desired color. The base colors, along with black and white, are combined in certain proportions within the printer's shop to achieve other PMS colors.
If it is very important to match a certain PMS color in your project, such as when a corporate logo color is used, you may want to suggest to the that printer purchase that particular color pre-mixed from the ink supplier. This will help ensure a close match. Another possible reason to buy pre-mixed PMS colors is if you have a very long print run, since it can be difficult to mix large amounts of ink and keep the color consistent through several batches.

A good example of some standard
Pantone formula guides
Certain PMS colors have to be purchased because they cannot be mixed, such as the Pantone metallics and fluorescent inks.
A common problem occurs when one tries to achieve the look of a PMS color while printing 4-color process (4CP). The only truly accurate way to use a PMS color in a 4CP project is to add the PMS as a fifth color to the job, which can become expensive. By definition, 4-color process uses only four inks: Cyan, Magenta, Yellow, and Black, (also known as CMYK), and therefore cannot match a PMS color which, by its nature, is composed of PMS base inks.
One compromise lies in Pantone's Process Color System, which attempts to simulate the PMS colors with CMYK inks. Once you have chosen your PMS color, you attempt to match it by choosing a similar 4CP color from Pantone's Process System Guide.
Another way of ensuring a match is to work backwards from a 4-color process piece. On the 4CP piece, pick a color you like, then pull out your PMS swatchbook and see what PMS color comes closest to the 4CP color you've chosen. Of course, this works only if you aren't locked into using a specific PMS color, like some corporate identity's state.