Rebecca there are a few different versions of windows and different licenses. I'll do my best to put it in simple easy to understand terms
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Upgrade = name says it all. It merely upgrades one version to the next IE: vista to win 7.
Retail = A Retail license you can install on one computer at a time. Meaning when it's activated... It can only be activated on one machine; however, you may install it on a Different or new computer provided that you Uninstall the OS (operating system) on the current computer.
OEM (builders license) = This license once activated is locked to the MOTHERBOARD only. You may replace any other component and still have a valid license. As cl-scott already said this normally will require a quick call to Microsoft activation line.
Hope this helps
Good luck with your new build. saw that your building yourself a gaming machine on a separate thread. Exciting. have fun!