This is an Online help format used by Microsoft in Visual Studio 2010. It’s the successor for Microsoft Help 2.
In 2008, Microsoft announced that they were working on a new help format to replace Microsoft Help 2. It shipped with Visual Studio 2010 Beta under the name Microsoft Help 3. In 2010, it was renamed to Microsoft Help Viewer 1.0.
Microsoft Help Viewer 1.0 was created anew. This format doesn’t use any of the Microsoft Help 2 code base.
It was a step forward in terms of user experience. For example, to view Microsoft Help 2 files, a separate application was necessary while the .mshc files can be viewed in a browser.
There’s a special taskbar applet in VS 2010 located in the tray that allows switching between offline and online help in the browser by pressing F1. It does that by resolving URLs to the corresponding topic pages.
Another useful feature included in VS 2010 and connected with Microsoft Help Viewer 1.0 is a library manager app. It manages help topics on the system – their download, installation, and uninstallation. Also, online help can be chosen as preferred when connected to the Internet.
MSHC is not a public format.