Globally Unique Identifier (GUID)
In Microsoft® Windows® installer, a 128-bit unique identification string used to identify different products, upgrades, and installer components. 12345678-1234-1234-1234-123456789ABC is a syntactically correct GUID. Microsoft® Visual Studio® Installer works with GUIDS that uniquely identify installer components, every version of every installed product, and all upgrades to product families.
Specifically, each installer component has a unique GUID. Also, every version of every product has a unique GUID. However, all upgrades of a product have the same GUID, which is the single upgrade GUID for that product.
Also known as a Universally Unique Identifier.