A transaction log backup operation backs up the transaction logs that contain records of all the committed and uncommitted transactions. Dirty pages are excluded during a transaction log backup operation. This backup is consistent and contains only the new changes since the last full backup or since the backup of another transaction log.
You can specify database log only backups to start when the space occupied in the log file reaches a specified percentage. For more information on automatic log backup schedule policies, see Creating an Automatic Schedule for Database Log Backups.
After a database failure, you can run the transaction log backup to recover data to the point of failure. You cannot perform a point-in-time restore from a full or a differential backup alone.
During a transaction log restore operation, if dirty pages are included, the completed transaction rolls forward. Any incomplete transaction that occurs during the time of the transaction log backup rolls back during the transaction log restore operation.