Google Docs, Sheets, and Presentations let you to view all changes made to documents by any collaborator. This revision history tool allows you to read and, if desired, revert to any earlier versions of your work. You can also see which collaborators made changes to any of these prior versions.
To see prior versions, use File > See revision history.
Restoring a previous version of a document doesn’t delete any of the versions of your document, it simply raises this version to the top of the revision history. All other versions of the document are kept, including the current version.
Click in the right pane to view your chosen version of the document. Any edits made by a particular collaborator will be shown in the body of the document and coded by color as detailed in the revision history panel. If you’d like to revert to the version you’re viewing, just click Restore this revision.
Click the X in the upper corner of the Revision History pane to return to the current version.
STOP PRESS! Google today launched Add-Ons for Docs and Sheets, and there’s an add-on specifically for tracking Document changes! Read about Add-Ons in this post.