Sandcastle, created by Microsoft, is a tool used for creating MSDN-style documentation from .NET assemblies and their associated XML comments files. The current version is the May 2008 release. It is command line based and has no GUI front-end, project management features, or an automated build process like those that you can find in NDoc. The Sandcastle Help File Builder was created to fill in the gaps, provide the missing NDoc-like features that are used most often, and provide graphical and command line based tools to build a help file in an automated fashion.
See also - MSDN: Recommended Tags for Documentation Comments (C# Programming Guide)