The SharePoint 2010 Managed Client Object Model is a set of .NET managed libraries that allow us to write code for client computers to work with many of the common objects in our SharePoint sites. Programs running on the client can add and remove lists, add, update, and delete list items, modify documents in document libraries, create sites, manage permissions of items, add and remove web parts from a page, and much more.
Open XML, the document format of Office 2007 and 2010, is an ISO/IEC standard (IS29500), which describes the internals of word-processing, spreadsheet, and presentation documents. Open XML files are stored per the Open Packaging Conventions specification (Part 2 of IS29500). They are essentially ZIP files that contain XML parts inside. The Open XML SDK is a .NET managed library that makes it easy to write programs to create, modify, or query Open XML documents. At the end of this article, I’ve listed a number of resources to help you get started using the Open XML SDK 2.0.
I've written an MSDN article, Using the SharePoint Foundation 2010 Managed Client Object Model with the Open XML SDK 2.0, which tells you how to work with Open XML documents that are stored in SharePoint document libraries.
That article is a follow-on article to Using the SharePoint Foundation 2010 Managed Client Object Model, which provides basic information about how to use the SharePoint Managed Client Object Model.