For those who aren't trained in printing or graphic design, it can all get very confusing. When you're talking about Spot Color you're pretty much talking about the Pantone Matching System.
Combinations of these colors, with the occasional use of PANTONE Transparent White, make up all the colors of the Pantone series. Kind of like paint chips at the hardware store, each color has a specific formula or recipe. A few key points to keep in mind:
Printing Inks are NOT opaque! They are slightly translucent, which means the color of the paper effects the color. So PANTONE Blue 072 looks one way on white paper, but on natural, off-white, or colored papers it would pick up some of the color of the paper.
Transparent White is NOT "white" ink.... but is actually CLEAR. As mentioned before, ink is translucent, so when you add "clear" it allows more of the paper to come through. This makes the color look like a lighter shade.
Each different color is a different ink, including Black. So if your project is PANTONE 286, PANTONE Red 032, and Black, than it is a 3 color job and requires 3 plates.
Process Color works a little differently. For "full color" printing, it uses 4 inks (Cyan, Magenta, Yellow, and Black) to produce a variety of other colors.
If you own an inkjet printer, or have ever used a color copier, you might already be familiar with this. With Process Color, the inks aren't mixed together to produce different colors as with Spot Colors. These colors are created with a series of half-tone dots in each of the 4 inks as needed to create the desired color. The inset to the right helps illustrate this point, and can generally be seen in any color older comic strip or comic book. Some magazines and art prints show it too, though the resolution tends to be higher, so you might need a magnifying glass of some sort.
