A SQL Server database is always comprised of data and log files. The data files contain the actual data whereas the log files contain a record of all changes made to the database. In a full database backup, both the data and the log files are written to the backup device.

This type of backup has one drawback; the transaction log is not truncated after it has been backed up. This means that the inactive part of the log containing already completed transactions is not deleted after the backup. There is a danger that the transaction log might fill up making it impossible to continue working with the R/3 System. In contrast, a transaction log backup on its own deletes the inactive part of the log and thus enables space to be re-used thus preventing continual growth of the log files.

Full database backups are made when the database is online and do not require you to stop the R/3 System. However, they result in a significant amount of disk I/O and should therefore be performed at a time when the workload is minimal.

A transaction log backup writes the log files of the database to the backup device. This enables the re-execution of transactions in the event of a database restore. When this type of backup is completed, the log is automatically truncated. This means log entries made prior to the oldest open transaction are deleted. Space in the log file can be re-used and in this way the continual growth of log files can be prevented.

Transaction log backups are made when the database is online. As they only contain the changes since the last transaction log backup, they are much smaller than database backups and therefore have less impact on performance.

A differential database backup only backs up the pages of the database that were modified since the last full database backup. If such a backup is used to restore a database it is worthless on its own, and can only be used in combination with the preceding full database backup. When multiple differential backups are created, each one records changes made since the last full database backup and no only those made since the last differential backup.