The Google Closure editor is a wrapper around the built-in browser editing capabilities. It is thus similar to other rich text editors like TinyMCE, CKEditor, etc. It is less feature-rich than either of those, but it's smaller and faster. The base editor is used by Gmail (most notably) and various other Google properties.
There is nothing within the public Google Closure editor to enable Google Docs style real-time collaboration. With that said, it has a plugin model which enables you to add new functionality. I would not recommend taking something like this on without a solid understanding of working with Google Closure.
Until recently, the editor was also used by Google Docs. However, the limitations of core browser editing technology became a barrier to innovation, so they built their own editing surface[1,2] (codenamed Kix). This editing surface is not included in Closure Library.
- http://googledocs.blogspot.com/2010/04/rebuilt-more-real-time-google-documents.html
- http://googledocs.blogspot.com/2010/05/whats-different-about-new-google-docs.html