Microsoft Word users can insert comment inside the document. This is useful when you want to provide some review comments or annotation with out making changes to the document. Word 2013 and Word 2010 users can insert comment by using the New Comment option available as part of the Review menu.
Add and delete comment in Word 2013 and Word 2010
Select the Word or sentence for which you want to insert a comment and click on the Review menu –> New Comment
If you want to delete a comment, select the comment and click on the Review menu –> Delete Comment
Screenshot from Word 2013
You also have option to clear all the comments in the document using Delete All Comments in Document. And if you want to quickly navigate between the comments then use the Previous and Next option available as part of Comments section.