DevShed introduces a new kind of class in their latest tutorial - bridge classes:
The bridge class, or what's commonly known as the bridge pattern, lets you create a class with its abstract functionality and implementation residing on different class hierarchies. This lets you decouple the class from its concrete application. This article, the first of three parts, introduces you to the bridge pattern and its uses.
They look at what it is first, describing it as a method of decoupling the functionality behind a factory object. This is followed (logically) with a simple example of one in use with the creation of a "Message" class and object. They put the core logic of the pattern in place behind it and combine the object and the backend to save the message off to a file.