The subkeys and registry values associated with the HKEY_LOCAL_MACHINE\SOFTWARE\Classes key contain information about an application that is needed to support COM functionality. This information includes such topics as supported data formats, compatibility information, programmatic identifiers, DCOM, and controls.

AppID Configuration options for COM-based applications.
CLSID Configuration options for COM classes.
<file_extension> Associates a file name extension with a ProgID.
FileType Used by GetClassFile to match patterns against various file bytes in a non-compound file.
Interface Associates an interface name with an interface ID (IID).
<ProgID> Identifies a class. Note that the ProgID is not guaranteed to be globally unique, unlike a CLSID.
<version-independent ProgID> Used to determine the latest version of an object application.