A type library is a special type of file that uses a standard format to describe the interfaces, classes, methods, properties, and events an application makes available through COM. Most applications that implement COM will provide a type library that describes the implementation.
Type library files have the file extension .TLB or .OLB. It’s also possible for type library information to be embedded into executable files or DLLs. When a COM-enabled application is installed, it typically adds an entry to the Windows Registry that tells the system where the type library information for the application can be found. This makes it easy for other tools, such as Dexterity, to find out what COM-enabled applications are installed on the current system.
The information in a type library is useful when you are developing an integration to a COM-enabled application. Development tools like Dexterity can use the information in the type library to verify the code you write. With this information, Dexterity can ensure that you use correct names for the properties and methods. It can also verify that the values used for COM methods and properties have the appropriate parameter types.