A .bak file is a backup copy of a file that is created by a program when it is about to overwrite an existing file. The .bak extension is used for general backup files, and many applications and computer processes use it to create backups of documents that the program is creating. The .bak naming scheme makes it possible to retrieve the original contents of the file in case of a failed write that corrupts the file, which could be caused by an operating system crash, power outage, or disk space exhaustion.

There is no standardized format for creating .bak files, and each program that creates them can determine how to format them. The most common .bak files are database backups of SQL Server databases, but other programs also create files with this extension. Some programs automatically create .bak files to back up users data, while others allow users to manually create them.

To open a .bak file, you need to know what program created it. If you have information about the files history, such as what program its from, you can locate the program in question to see whether there is a restore button for the file. If you dont have information on the files history, you will need to take steps to extract information from the file.