SQL Server 2005 suggests the extension .bak to be used, previos versions didn't suggest any extension at all.
There isn't any standard extension for backup files, and there is no registered file type for them. As Cakinci pointed out, SQL Server doesn't care at all what extension the file has when reading it.
We are not limited to 8.3 file names any more, all modern programs can handle long file names, so you could use the extension .backup if you like.
From my experience the .bak extension is more frequently used than .bck extension.
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by: CakinciPosted on 2009-01-24 at 04:33:41ID: 23456107
By default Sql server will define .bak extension to the backup files. But as in your condition you can change it to whtaever you want it to be. There will be no diffrenece in the backup file when you change the extension from the maintanance plan or form the sql backup wizard.
But keep in mind that if you will change the .bak extension to something else; while restoring the database using that extension be sure to select all files from the file type drop down menu that the wizards shows. Otherwise you will not see you backup file. Because by default it will only show you the by default extension files(.Bak)
I think microsoft done like this because for english users .bak extension is logical and resembles the word "backup". But in other scenario for the "turkish" people using sql server wants to give a resembling extension like "ydk" it means backup again.
So as a conclusion backup file is an extension free file.(independent)