What is it?
Windows Live Writer introduces the Provider Customization API that enables both rich customization of Windows Live Writer's behavior as well as the opportunity to add new functionality to the product. Currently Windows Live Spaces, WordPress, and TypePad have all taken advantage of this API to expose additional service-specific features within Windows Live Writer.
Description of writerbrowserextension.dll from Microsoft
“Windows Live Writer, developed by Microsoft, is a desktop blog-publishing application that is part of the Windows Live range of products. It features WYSIWYG authoring, photo-publishing and map-publishing functionality, and is currently compatible with Windows Live Spaces, SharePoint blogs, Blogger, LiveJournal, TypePad, WordPress, Telligent Community, PBlogs.gr, JournalHome, the MetaWeblog API, the Movable Type API, Blogengine, Squarespace, and all blogs that support RSD (Really Simple Discoverability).”