The Decorator pattern is a well known pattern written by the Gang of Four (GoF). You can find further information about this and other very useful patterns checking their book:
Design Patterns - Elements of Reusable Object-Oriented Software Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides Addison Wesley
This pattern provide an alternative to subclassing for extending functionality, that is, you can add responsabilities to an object that can be withdrawn later without using subclassing. The decorator acts as a wrapper that forwards requests to the decorated object and may optionally perform additional operations before or after forwarding the request.
I'm a big fan of the Head First series myself. You can review a sample chapter from their Design Patterns book here. As luck would have it, this chapter is all about the Decorator pattern. Hopefully you'll like this chapter so much that you'll go out and buy the book like I did!