Globally unique identifiers (or "universally unique identifiers") are used to identify COM interfaces, coclasses, type libraries, etc. Actually it is binary structure of type GUID, but can be represented as string of format {XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX}, where X are hexadecimal digits 0 to F. To get pointer to GUID structure, use either G{XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX} or uuidof(identifier) where identifier is name of interface or coclass. GUID-s for different types often are called differently (IID for interfaces, CLSID for classes, etc), but it is the same GUID. Often used REFIID actually is pointer to GUID of interface.