The co-authoring functionality in Office now makes it possible for multiple users to work productively on the same document without intruding on one another’s work or locking each other out.
You can use this new co-authoring functionality in PowerPoint 2010, Word 2010, and OneNote 2010 documents on SharePoint Server 2010. New co-authoring functionality is also supported in Excel Web App and OneNote Web App. Learn more about collaboration features in Word here: Collaborate on Word documents with real-time co-authoring
When you work on a file that is stored on a supported server, clicking Save does the following:
Saves any changes you have made to the file on the server
Updates (refreshes) the file you are viewing with any changes made by other authors
Stores the previous version
To compare the current version with a previous version of the document, click the File tab, click Info, and then click the stored version of the document. You can use the Compare feature on the Review tab to compare that version with the current version of the document.