I have a backup file of SQL Server version 2012 but I want to restore that backup on SQL Server 2008.
Is this possible ?
Yes, you cannot restore the backup of SQL server 2012 onto 2008 or lower versions because it is not supported way to do this.
However, there can always be a workaround by creating a blank database on 2008 and then use generate script wizard on 2012 to script out the schema and data or with help of third party tools to get the same.
The best would be look at the two options provided by @Kin in his excellent answer at Restore SQL Server 2012 backup to a SQL Server 2008 database?
Additionally you may want to refer here