BAK File

In computing, ".bak" is a filename extension commonly used to signify a backup copy of a file.
When a program is about to overwrite an existing file (for example, when the user saves the document he or she is working on), the program may first make a copy of the existing file, with .bak appended to the filename. This common .bak naming scheme makes it possible to retrieve the original contents of the file. In a similar manner, a user may also manually make a copy of the file before the change and append .bak to the filename.
Other naming schemes are also in widespread use: file~, file.orig, file.old, and so on.
Database Applications like FoxPro and SQL Server use .bak files to back up their databases.^{[citation needed]} Many applications, like XML shell, create .bak files in their Autosave process.^{[citation needed]} They don't get automatically deleted, so they need to be manually deleted after the process using it is stopped.

Applications that use bak files

• Finale Notepad
• FoxPro
• Sony Vegas
• SQL Server
• XML Shell
• AutoCAD

Thanks to :
Wikipedia