The Component Object Model (COM) Standard

COM is a standard that defines how objects and users of those objects interact. As this is a binary standard, many different ActiveX objects can use objects defined within COM, regardless of the language used. An ActiveX control, for example, can work with a Visual Basic or a C++ application.

COM objects are manipulated by an interface, which is simply a set of methods that can be called and properties that you can set or get. Most objects have many different interfaces. The most important of these interfaces is the IUnknown interface, which all COM objects implement and all other interfaces inherit. Although IUnknown is the only interface that all objects must support, most objects also support the IDispatch interface. Objects that support the IDispatch interface are called automation servers.

A type library is a special file (which usually has a .tlb extension but can also be imbedded in an executable or dynamic link library file) that contains a description of an object and its interfaces. Some of the information that this file contains includes method names, parameters, and parameter types.