An add-in is a program that's attached to an application to give it additional functionality. To attach an Excel add-in, use the Add-Ins tab in the Excel Options dialog box.
In addition to the add-ins that ship with Excel, you can download additional add-ins from Microsoft's Web site (http://www.office.microsoft.com), and you can purchase or download many third-party add-ins from online services. You can use the coupon in the back of the book to acquire a free copy of the Power Utility Pak add-in. And, as I detail in Chapter 21, it's very easy to create your own add-ins.