The Activity Stream shows a history of who modified an article and when. The activity stream can contain notes about changes that were made to an article as well. The Activity Stream is available in the web application only.
Revision notes are notes that can be added to an article (or specific content blocks within the article) in order to communicate with other authors about changes that need to be made. Revision notes can be created in the web application. Revision notes can be viewed in the web editor or desktop editor.