Spot colors are vendor specific colors. These colors are created by a vendor such as Pantone or Toyo, among others.
Spot colors are built using the vendor's ink and designed to always provide the same color on press if the same vendor is used. For example, you can specify a Pantone 185C to someone and they can look up and see that Pantone 185 is a specific red, and the C designates coated stock (paper) to be printed on. Use of spot colors is designed to ensure colors are always accurate and never vary across various projects. If you purchase Pantone 185 ink, it will always be the same color of red. Many companies utilize spot colors for logotypes and specific brand colors to ensure the colors are always the same.
In terms of production, spot colors are output to their own plates for press/imagesetters because they are designed to be printed with the specific ink designated in the file. They do not separate to CMYK unless specifically told to.
Global colors are merely an application setting telling Illustrator that you want that color to always be the same build whenever it is used. Basically, act like a spot color in the app, but output like a processes color. When global colors are output, they are separated into their CMYK (or RGB) build just as if they were standard process colors. The "Global" aspect in Illustrator is really just a way to streamline the use of color and retain some continuity while working with processes colors.