File Format References
The Microsoft Excel™ Binary file format (BIFF5 and BIFF8) is a binary file format that was used by Microsoft Excel™ between versions 95 and 2003. The format is supported (to various extents) by most spreadsheet programs, and by PHPExcel's Excel5 Reader and Writer. BIFF files normally have an extension of .xls. Documentation describing the format can be found online at http://msdn.microsoft.com/en-us/library/cc313154(v=office.12).aspx or from http://download.microsoft.com/download/2/4/8/24862317-78F0-4C4B-B355-C7B2C1D997DB/[MS-XLS].pdf (as a downloadable PDF).
Microsoft Excel™ 2007 shipped with a new file format, namely Microsoft Office Open XML SpreadsheetML, and Excel 2010 extended this still further with its new features such as sparklines. These files typically have an extension of .xlsx. This format is based around a zipped collection of eXtensible Markup Language (XML) files. Microsoft Office Open XML SpreadsheetML is mostly standardized in ECMA 376 http://www.ecma-international.org/news/TC45_current_work/TC45_available_docs.htm and ISO 29500.
This is the OpenOffice.org XML File Format for spreadsheets. It comprises a zip archive including several components all of which are text files, most of these with markup in the eXtensible Markup Language (XML). It is the standard file format for Libre Office, OpenOffice.org Calc and StarCalc, and files typically have an extension of .ods. The PHPExcel OOCalc Reader recognises this format. The published specification for the file format is available from the OASIS Open Office XML Format Technical Committee web page (http://www.oasis-open.org/committees/tc_home.php?wg_abbrev=office#technical). Other information is available from the OpenOffice.org XML File Format web page (http://xml.openoffice.org/general.html), part of the OpenOffice.org project. https://www.oasis-open.org/standards#opendocumentv1.2
Microsoft Excel™ 2003 included options for a file format called SpreadsheetML. This file is a zipped XML document. It is not very common, but its core features are supported by PHPExcel's Excel2003XML Reader. Documentation can be found at http://msdn.microsoft.com/en-us/library/aa140066%28office.10%29.aspx though it’s sadly rather sparse in its detail.
This is the Microsoft Multiplan Symbolic Link Interchange (SYLK) file format. Multiplan was a predecessor to Microsoft Excel™. Files normally have an extension of .slk. While not common, there are still a few applications that generate SYLK files as a cross-platform option, because (despite being limited to a single worksheet) it is a simple format to implement, and supports some basic data and cell formatting options (unlike CSV files).
The Gnumeric file format is used by the Gnome Gnumeric spreadsheet application, and typically files have an extension of .gnumeric. The file contents are stored using eXtensible Markup Language (XML) markup, and the file is then compressed using the GNU project's gzip compression library. http://projects.gnome.org/gnumeric/doc/file-format-gnumeric.shtml