A backup is a copy made of files or data found on a computer which has been made in case the original file becomes corrupted or unusable.
Applications on a computer also create backups of files usually when changes have been made, these files use the .bak extension to identify them as backups of a file and are normally created by various programs but can be made manually by adding the .bak extension to the file name.
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How to open .bak files?
Software to open .bak files
What are .bak files