BAK files, also known as backup files, are used by a huge number of applications and processes to create backups of existing data, often as part of an auto-save feature.
There is no standard format for these files, as the data within them is often a copy of any kind of data.
It is often possible to safely remove these files as they are simply there as a backup of existing data. However, we do not recommend that you remove these files unless you are sure that it is safe to do so.
If you are trying to find which application created the backup file, the .bak file's location can often provide clues.