Recall the question posed at the beginning of Chapter 1: What is a database? The answer given there was:
A database is a tool that stores data, and lets you create, read, update, and delete the data in some manner.
This broad definition allows you to consider all sorts of odd things as databases including notebooks, filing cabinets, and your brain. If you're flexible about what you consider data, this definition includes even stranger objects such as a chess set (which stores board positions) or a parking lot (which stores car types and positions, although it might be hard for you to update any given car's position without the owner's consent).
This chapter moves into the realm of computerized databases. Relational databases are by far the most commonly used computerized databases today and most of this book (and other database books) focus on them, but it's still worth taking some time first to learn a bit about other kinds of computerized databases that are available. Relational databases are extremely useful in a huge number of situations but they're not the only game in town. Sometimes a different kind of database may make more sense for your particular problem.
Before you start frantically throwing tables together, building indexes, and normalizing everything in sight, it's worth taking some time to study some of the other kinds of databases that are available.
This chapter describes different types of databases including flat files, spreadsheets, ...