FileInfo.com - The File Extensions Resource

.FBK File Extension

File Type 1	Microsoft Dynamics NAV Backup File (Most common)
File Format	File
Common?	No
File Description	Backup of a database created with Microsoft Dynamics NAV; contains financial data used for business management and analysis; can be used to restore data if the primary database (.FDB) file becomes corrupted or is deleted. Microsoft Dynamics NAV was previously called Microsoft Navision.
Program(s) that open fbk files	Windows: Microsoft Dynamics NAV or Navision

File Type 2	Family Tree Maker for Windows Backup File
File Format	Proprietary File
Common?	No
File Description	Compressed backup of a family tree (.FTW) file created with Family Tree Maker for Windows; may be created as a backup file in case the original family tree file is damaged or deleted; can be restored by selecting "File->Restore From Backup" within Family Tree Maker. NOTE: Family Tree Maker for Windows uses the ".fbk" file extension, while Family Tree Maker for DOS saves backups as .FBC files.
Program(s) that open fbk files	Windows: Ancestry.com Family Tree Maker for Windows

Information from FileInfo.com - The File Extensions Resource.
http://www.fileinfo.com/extension/fbk