Spot color refers to a color achieved on press with an individual, specially-mixed ink.
Usually a spot color is used to produce a color different from what can be produced using normal CMYK inks. In contrast to CMYK or process color, which creates colors by laying down layers of cyan, magenta, yellow, and black in varying amounts on the printed page, spot colors are pre-mixed and applied individually to the printed page. The general use of spot colors is to produce colors that are outside the gamut of process colors (for example, more saturated). A spot color requires a dedicated plate and printing unit.
The Pantone Matching System is the dominant spot color printing system in the United States. Prior to PANTONE's wide adoption, vendors often had their own color call-outs or nomenclatures. The PANTONE matching system is based on 14 "base" inks that are used to mix all the possible colors in the system. The "base" inks are as follows:
The PANTONE Formula guide contains the "recipes" for all the colors in the PANTONE matching system. PANTONE is only one of many color systems. Others include TOYO, TRUMATCH, & FOCOLTONE.
(Don't try to use spot colors in newspapers for instance. The chances of the press operator loading another plate just for your logo on page 27 is remote indeed.)