The PHPExcel class is the core of PHPExcel. It contains references to the contained worksheets, document security settings and document meta data.
To simplify the PHPExcel concept: the PHPExcel class represents your workbook.
Typically, you will create a workbook in one of two ways, either by loading it from a spreadsheet file, or creating it manually. A third option, though less commonly used, is cloning an existing workbook that has been created using one of the previous two methods.
#### Loading a Workbook from a file
Details of the different spreadsheet formats supported, and the options available to read them into a PHPExcel object are described fully in the PHPExcel User Documentation - Reading Spreadsheet Files document.