Microsoft COM (Component Object Model) technology in the Microsoft Windows-family of Operating Systems enables software components to communicate. COM is used by developers to create re-usable software components, link components together to build applications, and take advantage of Windows services.
The following COM Spec is from the CD of the 1997 book Inside COM by Dale Rogerson. I’m surprised that the CD is still working. Here is the download link:
This Ole New Thing blog also illustrates the memory layout of a COM object.