Link Recommendation: Periodization

Here is a very basic, but easy to understand explanation of the concept of periodization.

Basic Periodization I
It is hosted on Elite FTS and was written by Simon Perreault.

In periodization, you vary your work load in a workout in order to maximize your gains. You don't simply lift the same weights over and over; you need to create some form of disruption of homeostasis. In other words, you have to change it up in order to get your body to change by getting stronger.

If you want to know more about periodization, you need to check out Practical Programming for Strength Training by Mark Rippetoe, Lon Kilgore, and Glenn Pendlay.