On the KillerPHP.com website, there's a brief look at design patterns, specifically using them in PHP:

Since the release of PHP 5, the PHP world has slowly started to get into more advanced programming, things like object oriented concepts and the use of design patterns.

Stefan Mischook describes the patterns simply, as nothing more than standardized ways of solving common programming problems. He lists a few of the more well-known patterns and answers a few common questions developers have about them:

• When should you start learning and using design patterns?
• The dark side of design patterns (when should I use them?)
• What can we learn from Java's bad experience?

I love his final comment - it sums the design pattern experience up well:

I am not saying that design patterns are bad, I just think they should be used only in the right context: you don't need an 18 wheeler truck to move a chair...