The overview is here:
Visual Studio 2005 Tools for the 2007 Microsoft Office System (Visual Studio 2005 Tools for Office Second Edition or VSTO 2005 SE for short) is a free add-on to Visual Studio 2005 that empowers developers to build applications targeting the 2007 Office system. Developers can now harness the benefits of the 2007 Office system platform and create Office-based solutions using the professional development environment of Visual Studio 2005.
Anyone who has a licensed version of Visual Studio 2005 Tools for Office or Visual Studio Professional and above (either standalone versions or via an MSDN Premium or Professional subscription) is eligible to download a free copy of VSTO 2005 SE.