Open XML Format

Open XML SDK 2.5
This file format is based on XML and Zip technology
'' officeopenxml.com/ssstyles.php

[Content_Types].xml

Used to identify all the separate files that make up the office document.
Parts need to have identifiable content types.

[_rels]

This folder contains information about the relationships between all the different parts.

[docProps]

This folder contains three files which contain information about the document properties.
app.xml - the unique properties specific to the document at the application level, such as number of pages, version etc
core.xml - the standard document properties
custom.xml - the custom document properties

[customXm]

This folder contains

[Xl]

This folder contains all information about the workbook

link - msdn.microsoft.com/en-us/library/office/bb448854.aspx
link - blogs.msdn.microsoft.com/brian_jones/2006/11/02/simple-spreadsheetml-file-part-1-of-3/
link - blogs.msdn.microsoft.com/brian_jones/2006/11/15/simple-spreadsheetml-file-part-2-of-3/
link - blogs.msdn.microsoft.com/brian_jones/2007/05/29/simple-spreadsheetml-file-part-3-formatting/
link - github.com/davecra/OpenXmlFileViewer