When you're creating a workbook and entering data in it, you'll probably be clear about which data is which, why you're laying the data out in the way you've chosen, and why you've used particular types of charts or decided to construct formulas in an idiosyncratic way.
But someone else who opens your workbooks may not intuitively grasp how you've done things. So when you create a workbook you plan to share with others, or when you prepare a workbook for sharing, it's a good idea to document the workbook. This can save you time, bewilderment, and any number of infuriating questions in the days, weeks, or months to come.
Excel provides three main tools for documenting your workbooks: