In Microsoft Exchange, a transaction log is a file that contains a record of the changes that were made to an Exchange database. Information that needs to be added to a mailbox database is first written to an Exchange transaction log and then the contents of that transaction log are later written to the Exchange Server database.
Microsoft chose to use the transaction log model so users can still access Exchange Server during a backup. If data were written directly to the Exchange database, the administrator would have to close the database before it could be backed up, which means the Exchange Server would be unavailable to users during that period of time.
See also: circular logging