Spot colors are specific individual colors that can be used alone, or in combination with other colors, including process colors. Each spot color used will be printed with the appropriate color ink on press. There are many different spot colors and color libraries to choose from. The PANTONE® (PMS, or Pantone Matching System) color libraries are some of the more widely used color libraries in the printing industry. 

Colors selected from color libraries are specified by their reference number instead of a color name. For example, the reference of PANTONE 300 can be used instead of the color name Red. Other color libraries include FOCALTONE, TOYO, and TRUMATCH.

The important point to make here is that selections from a color library are used as separate and unique spot colors. If you use, for example, ten spot colors from the PANTONE library, you will have ten separate color inks on press. The goal, of course, is to build your document pages with only the appropriate number of colors for press.
Spot Colors as Color Builds

You can use any spot color as the basis for a CMYK color build. Simply convert your selected spot color the to the equivalent CMYK values. However, not surprisingly, there is one consideration.

Similar to how the CMYK color space cannot accurately represent all RGB colors, the CMYK color space cannot accurately represent all spot colors. You only need to look at a Pantone Color Bridge Guide® to compare any PANTONE spot color next to their equivalent CMYK build values to see the differences.
Redefined Spot Color Values

As long as you are printing your specified colors as spot colors, then it will not matter if you change the CMYK values (Edit Color). However, if you are planning on converting all your spot colors to CMYK, and you have redefined your CMYK values, then you will be altering the actual color separations, and they will no longer match any printed color guide books.

For example, if you define a color like PANTONE 300 and are planning to print this color as a spot color, you can change the color values all you want and still get the same PANTONE 300 color on press. Spot colors are specific ink colors that have predefined ink colorant values which cannot be changed.

If on the other hand, you use the same PANTONE 300, then decide to convert it to CMYK for press, and previously changed the CMYK color values, you will get a different resulting color then what is in the printed PANTONE color guide.
When To Use

Use spot colors when you have specific color needs, like matching logo colors or corporate identity pieces. When the color is just as important as the image itself, like team logos, you can use spot colors. More importantly, anytime you want a color to print consistently from page to page like in a banner or header, or from project to project, use a spot color instead of process color builds.

Some CMYK color builds are made without any 100% colors, and therefore all colors are screened. When a color is all screened and applied to text or other thin stroked graphic elements, it will produce a slightly soft or jagged edge. To produce a crisp sharp edge, use a spot color to replace the screened color builds.