COM
Component Object Model

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COM is a specification
It specifies how to build components that can be dynamically interchanged.
COM provides the standard that components and clients follow to ensure that they can operate together.
The COM specification is a document that sets the standard for the component architecture.
COM Components
COM component consists of executable code distributed either as Win32 dynamic page link libraries (DLLs) or as executables (EXEs).
Components written to the COM standard meet all requirements for component architecture.
COM components, page link dynamically using DLLs.
COM components can be encapsulated easily because:
COM components are fully language independent. They can be developed using almost any procedural language Ada to C to Java to Modula-3 to Oberon to Pascal.

Any language can be modified to use COM components, including Smalltalk and Visual Basic.
COM components can be shipped in binary form.
COM components can be upgraded without breaking old clients. Because it provides a standard way to implement different versions of a component.
COM components can be transparently relocated on a network. A component on a remote system is treated the same as a component on the local system.
More about COM
COM is not a computer language.
COM tells how to write components.
COM is not primarily an API or a set of functions like the Win32 API.
COM provides a way to develop language-independent component libraries, but COM does not provide any implementation.
COM Library
COM library, provides component management services that are useful for all clients and components.
The COM library also saves developers time implementing their own components and clients.
Most of the code in the COM library is support for distributed or networked components.
This reduces to write the networking code.