As a standard you can implement the following file extension naming convention for Microsoft SQL Server database backup files:

/* File extensions for SQL Server backups

Full Database Backup      - *.bak

Transaction Log Backup    - *.trn

Differential Backup       - *.dif

Filegroup Backup          - *.fil

*/

NOTE: file extension or filename extension are frequently misspelled as extention.

-- SQL Server full database backup basic syntax
BACKUP DATABASE [AdventureWorks] TO  DISK = N'F:\data\backup\AW.bak'
GO
-- SQL Server full database backup with options
BACKUP DATABASE [AdventureWorks] TO  DISK = N'F:\data\backup\AW.bak'
WITH NOFORMAT, NOINIT,  NAME = N'AdventureWorks-Full Database Backup',
SKIP, NOREWIND, NOUNLOAD,  STATS = 10
GO
------------
-- SQL Server transaction log backup
BACKUP LOG [AdventureWorks] TO  DISK = N'F:\data\backup\AW_20120201_1430.trn'
GO
------------
-- SQL Server differential backup
BACKUP DATABASE [AdventureWorks] TO  DISK = N'F:\data\backup\AW.dif'
WITH  DIFFERENTIAL
GO
------------

Related article:

SQL Server 2000 Backup and Restore