Globally unique identifiers (or "universally unique identifiers") are used to identify COM interfaces, coclasses, type libraries, etc. Actually it is a 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 uuidof. GUID sometimes is called differently (IID for interfaces, CLSID for classes, REFIID, etc).