Anyone interested in Mac OS X development should check out the latest Xcode article on the Apple Developer Connection, Understanding Xcode Projects:
To become truly proficient with Xcode, whether you’ve been using it for a while or are just starting to make the transition, an understanding of the Xcode user interface and how it is used to organize a software project is required. This understanding will take you a long way towards understanding the philosophy behind Xcode and how it works, and will help to make it work better for you.
In short, the article introduces what we call the Xcode project model, which defines how Xcode projects are organized, how Xcode generates products from targets, and how projects and targets relate to one another.
It doesn’t cover the details of the build settings that are used by targets or how they’re collected in target and project configurations. Both topics are covered in depth in the Xcode User Guide. I also wrote about configurations when they were introduced in Xcode 2.1 as a replacement for build styles.
Update: The article Working with Xcode Build Settings was recently posted covering configurations and build settings.