Is the structure of SQL Server database backup BAK file similar to NTBackup BKF file? Is there any reference which can be redirected to understand the difference between NTNAckup and SQL Server Backup (*.bak) files?
No, the SQL format and the NT format have absolutely nothing in common.
For an overview of SQL Server backup semantics, see Backup Overview (SQL Server).While the NT backup format is documented at NT Backup File and there is an API for access, the SQL’s backup format is undocumented and the only access API is via the T-SQL BACKUP and RESTORE statements. The SQL backups (format, semantics, API) long predates the existance of NT as an operating system.