The Open XML SDK for Microsoft Office is built on top of the System.IO.Packaging API and provides strongly typed part classes to manipulate Open XML documents.
Open XML is an open ECMA 376 standard and is also approved as the ISO/IEC 29500 standard that defines a set of XML schemas for representing spreadsheets, charts, presentations, and word processing documents. Microsoft Office Word 2007, Excel 2007, and PowerPoint 2007 and later versions all use Open XML as the default file format.
The Open XML file formats are useful for developers because they use an open standard and are based on well-known technologies: ZIP and XML.
The Open XML SDK for Microsoft Office is built on top of the System.IO.Packaging API and provides strongly typed part classes to manipulate Open XML documents. (Use version 2.5 whenever possible as it fixes some problems in version 2.0.) The SDK also uses the .NET Framework Language-Integrated Query (LINQ) technology to provide strongly typed object access to the XML content inside the parts of Open XML documents.
The Open XML SDK simplifies the task of manipulating Open XML packages and the underlying Open XML schema elements within a package. The Open XML Application Programming Interface (API) encapsulates many common tasks that developers perform on Open XML packages, so you can perform complex operations with just a few lines of code.
Above is taken from the Overview section here.
In 2015 the Open XML SDK was made open source, public on GitHub. Version 2.6.0 was released which fixes a bug in the way it works with System.IO.Packaging. It is otherwise the same as 2.5. More details are available here.
