A differential backup only copies changes since the last full backup.  Since the data on most computers changes very little during day-to-day use, the time to complete the backup is smaller than running a full backup every time.

Differential backups offer a good compromise between the space-saving advantages of incremental backups, and the faster restore times of full backups.  If data needs to be restored, the backup software will need to process, at most, two backup media – one full backup and one differential backup.