Tail-log backups are new to SQL Server 2005. The tail-log backup captures records on the transaction log that were written since the last transaction log backup. If you’re going to restore a database to the point of failure, then you need to take a tail-log backup before you start the restore operation.

If you’re going to restore to a point in time prior to the last transaction log backup, if you’re moving the database from one server instance to another, or if you’re overwriting the existing database, then you won’t need a tail-log backup. If the transaction log is damaged and you can’t take a tail-log backup, then you must restore without one.

Learn more from "Backup and Restore Fundamentals" and "Advanced BACKUP and RESTORE Options."