Backing up a database makes a copy of a database, which can be used to restore the database if it is lost. Backing up a database copies everything in the database, including any needed portions of the transaction log.

The transaction log is a serial record of all the modifications that have occurred in a database, and which transaction performed each modification. The transaction log is used during recovery operations to roll forward completed transactions, and roll back (undo) uncompleted transactions. 

Backing up a transaction log backs up only the changes that have occurred in the transaction log since the transaction log was last backed up.

A backup operates like a fuzzy snapshot taken of a database or transaction log: 

• A database backup records the complete state of the data in the database at the time the backup operation completes.

• A transaction log backup records the state of the transaction log at the time the backup operation starts.

Note: Transaction logs are not backed up or truncated during full or differential database backups. To back up and truncate Transaction logs, you must perform a separate Transaction log backup. When you perform the Transaction log backup, select the Remove inactive entries from Transaction log option to truncate the log files.