The extension BAK is used by numerous programs to mark their backup files.
These files do not have a uniform format. The content depends entirely on the generating program. Any type of data may be included in the file.
Many programs use BAK files to back up an older version of a user-edited file. These files are usually only open with the program that created them.
The directory in which the file is located can provide an indication of which original file has the file and with which program it can be opened.
Exceptions are files in which the suffix BAK has been appended to the original end, e.g. textdatei.txt.bak. Then of course it suffices to remove the ending bak.