As explained in the "Problem," there are many issues to solve and a few critical design decisions that must be taken before proceeding with the rest of the site, as they create the underpinnings used for all further development. It would be too late to design architecture to support multiple data stores once you've already developed half the site. Similarly, you couldn't decide on a strategy for handling configuration settings, such as database connection strings, after you wrote an entire module that uses the database. If you ignored these issues and started coding main modules first, you would end up with a poor design that will be difficult to correct later. As with this entire book, many of the new ASP.NET 2.0 classes and controls are utilized, as they apply to each section of code we develop, and sometimes these new features will provide a complete and ready-to-go solution to a particular problem.
If you've been doing any software development in recent years you should be familiar with the multi-tier software design (also called n-tier design). To recap it in a few words, it divides the functionality, components, and code for a project (not just a web-based project, but also WinForm and other types of projects) into separate tiers. There are generally four of them:
Data Store: Where the data resides. This can be a relational database, an XML file, a text file, or some other proprietary storage system.
Data Access Layer: ...