What is masking?

Masking is a technique for including and excluding image elements. Whilst this entry is not by any means a tutorial, it does, (I hope), allow the reader to consider the possible applications of this way of working.
Do you remember, early in the Photoshop days, that the idea of "swapping heads" seemed like a big deal?
Have you ever wondered how easy it is to do?
The truth is that swapping heads is one of the simple masking / clipping tricks. It gets much harder than that.
To illustrate a fairly straight-forward case of masking, I offer the following images. First the original, then the "extracted" subject, the new background, and finally, the end product.
Please have a look:

Like it?  Want this type of work in your image? cremetti@cremetti-commercial.com
Please let me know your thoughts, and feel free to ask questions.