Here is a video tutorial on Microsoft Excel that demonstrates the concept of a "Workbook" and outlines the advantages of working with multiple worksheets instead of crowding your workings onto a single worksheet.
The video shows how, as a typical example, a Workbook is created to represent the entire budget for an organisation, and where each Worksheet is just one component of that budget.
The video also discusses how to create a print out of your workbook with each worksheet given a sequential page number and other features to give an overall professional appearence.
The following link also provides an Excel Workbook that was used in the demonstration in the video.