This new OOP world is just full of new terminology and concepts! Let's take a look at polymorphism—what it means and how it can help you in your development efforts.
Simply put, polymorphism means that multiple objects will provide the same interface elements. For example, you might create a set of objects for your business that all provide a "standard" set of file functions. All of these objects would provide related methods and properties with predictably identical names, such as FileName, Read, and Write. This would simplify, standardize, and coordinate these objects, making them easier to use. It also would make them more predictable and consistent.
Visual Basic handles polymorphism a little differently than C++ does, for instance. Historically, in most OOP languages, the methods and properties of one object are inherited by a new object. The new object can then optionally modify the inherited interface elements or expose them "as is" to client applications.
Visual Basic, on the other hand, doesn't use inheritance to provide polymorphism. Instead, Visual Basic lets us create an interface, which as mentioned is a set of related property and method definitions, contained in an abstract class. An abstract class is simply a class module containing empty, ghost-like definition shells of methods and properties. This special class is then declared in class modules that agree to implement these interface items using the Implements keyword.
I like to think of the abstract class as a kind of contract. Multiple class modules can each agree to abide by this shared contract by using the Implements keyword. A client application can then declare instances of these objects, including an object variable defined by the special abstract class. By accessing each object's implemented interface elements through the abstract class-defined object, it's guaranteed that each object will handle the agreed-upon method or property in its own way.
The best way to master these concepts is to try them and work with them. The example provided in the Books Online documentation provides a great way to experiment and learn how they work. Search Books Online for Tyrannosaur, or if you're in a hurry or don't feel like typing that much, search for Flea. The explanation of how a Tyrannosaur object and a Flea object each agree to implement a Bite method through Visual Basic's form of polymorphism is a great way to learn—and to remember—how this all works.