An interface can be defined as a named collection of abstract members. Because an interface does not provide any implementation details, it is common to regard an interface as a behavior that may be supported by a given type. When two or more classes implement the same interface, you are able to treat each type the same way (via interface-based polymorphism) even if the types are defined within unique class hierarchies.
VB 2005 provides the Interface keyword to allow you to define a new interface. As you have seen, a type can support as many interfaces as necessary using the Implements keyword. Furthermore, it is permissible to build interfaces that derive from multiple base interfaces.
In addition to building your custom interfaces, the .NET libraries define a number of frameworksupplied interfaces. As you have seen, you are free to build custom types that implement these predefined interfaces to gain a number of desirable traits such as cloning, sorting, and enumerating. Finally, you spent some time investigating the stock collection classes defined within the System.Collections namespace and examining a number of common interfaces used by the collectioncentric types.