File Extension BAK
File extension BAK is used by a very large number of different applications to store BAcKup data. Backups are redundant copies of some data which are stored in a different location than original. Good backup practice is to use different physical media at a different physical storage location.
Backups are also created before executing complex operations that have high likelihood of failure. If some error occurs it is then possible to restore data to a previous consistent state and retry the opearation again.
BAK files can usually be opened only by the application that created them. Sometimes BAK file can be just copied over the original file by changing the file extension from BAK to the appropriate one. Such opeartions should only be executed by skilled professionals since changing file extensions can cause data errors and corruption.