Starting with Visual Studio 2005 (except Express ed.) you can create and modify classes using Class Designer tool with two-way synchronization between the diagrams and the code. And Designer-created class diagrams are stored in a file with .CD extension, in XML format. However, this .CD file is not a self-contained one, meaning, you cannot open the file without the associated class files. If you actually open the CD file in Notepad, you can see references to the source code files and other positioning coordinate details, etc required to render the classes in designer mode. So, the moral of the story is that you cannot send just a .CD file to your co-developer thinking he can see your class design. He needs the complete source code too. In other words, the Visual Studio Class Designer is more of a visualizing tool (or a tool to create classes visually) than a diagramming tool by itself.
Now MSDN has come up with its own version of online code repository website - the MSDN Code Gallery. This site feels a lot like CodePlex but with the exception of code versioning/project management support. As Soma says, there is no TFS or alike support in MSDN Code Gallery - it's just a place for the dev community to share code samples.
Some cool samples are already in. Check them out!