What is the difference between a Process Color and a Spot Color?
MAM-A gives great definitions for these terms on this webpage.
Pantone's description focuses on the difference as it relates to the Pantone color system. Pantone has both process colors and spot colors, but they differ quite clearly; this webpage can help you see the difference
This Color Tutorial from the University of Missouri Printing Services offers great information about process colors, their limitations, tints, etc.
Not sure when to use a spot color or a process color? About.com offers this article to help differentiate when to use which color mode.
I hope this helps in your understanding of which colors to use for printing.