The QBK file is a QuickTax backup file created by the Intuit Canada tax filing software, QuickTax. Although these files are generally not needed, they are saved with a corresponding tax file (often saved as a Qyy, or Q08 for 2008 that contains the original tax return). If the tax return file is corrupted, it is a simple matter of renaming the backup file that corresponds with your tax return from a QBK to a Qyy format in order to retrieve your backed up tax return.