The extension .BAK refers to files of the type, generic backup files. These are the files created by Windows based software on the PC that want to store the database backup information relating to the user's data.

The BAK files are created automatically by the software and contain the contents of the original documents. The user can use the backup files as the original content just by removing the BAK extension and adding the relevant file extension to access the data. Many a times, the auto save files also have .BAK extension. The Backup files are extremely useful in system restore and upgrade process.