Auto recovery and Automatic back up in Microsoft Word.
Automatic back up
Not only can Microsoft Word save your document to restore the complete document if your computer system crashes before you have a chance to save your work, but also it can save a copy of the original document.
To do that you have to set file-saving features in Microsoft word by clicking The Tools, Options, click Save tab then select or click Always create a backup copy box and select or click Save Autorecover info every (set your desired time in minutes) in Save options.
The original document is safely stored as long as you set that option.
The .bak extension is the file created by Microsoft Word when you make a new document in Microsoft Word. It means, even the power goes out, you still have a copy of the document that you create it.