Formula Functions: Your Building Block in Salesforce.com Formulas
Without hesitation, I would identify this chapter as one of the most important in this book. Along with the first chapter on the Salesforce.com data model, it will serve as the foundation for many of the upcoming chapters, as formula functions are used in a number of development areas within the Force.com platform. If you understand how to properly build and apply functions within Salesforce.com, you will be able to develop successful custom solutions for yourself, your employer, and your clients.
Functions can be used in a number of different areas: