COM InfoCommon Object Model is Microsoft's architecture for interchangeable, plugin objects. This is built on existing Microsoft Dynamic Link Library (DLL), Dynamic Data Exchange (DDE) and Object Linking & Embedding (OLE) technologies.
Active X is Microsoft's catch-all name for technologies that leverage COM; it is also driving the way to Distributed COM (DCOM), through new Intenet and Web technologies.
Other ReferencesGrafman's Active X Tips
Microsoft's Active Platform Info Page
Microsoft's COM/DCOM Info Page
Microsoft's COM spec
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