The BAK extension is used for files that can generally be opened by the software application that created them. BAK also means a backup file. It is a copy of any document on your PC. Software programs make them from the ones they overwrite at installation time. Backup files are also created during editing. If you have Excel or Word configured for automatic backup, a copy of the file being edited will be created from time to time. The copy is created while you edit the original and will have the extension BAK. Now if the app gets corrupted, you will have a ready copy.
The only difference between these extensions is that BUP is generally used as a backup for DVDs and BAK for other documents.