Transaction log backups capture records written to the transaction log file(s) defined for a database. The full and bulk-logged recovery models are the only models that support transaction log backups. These models cause transaction events to be retained in the transaction log so that they can be backed up. Simple recovery mode causes the transaction log to be truncated periodically and thus invalidates the usefulness of the transaction log backups.
The transaction log backups and their strong ties to the recovery model are discussed in more detail in the next section.