When taking backups of computers there an many different settings you can use to make the backup time more efficient, so the backup runs a quickly as possible.
A full backup is required regularly, but not all the time. By using an incremental backup or differential backup it uses the full backup to only backup the changes.
An incremental backup will backup only the changes since the last incremental backup. So it uses the full backup, then writes the changes to a new incremental file. Next backup will use the full backup and all the incremental files to write only the changes to a new incremental file. So you need to have the full and all the incremental backups in tack to do a restore.
An differential will backup all the changes since the last full backup, and write these changes to a new differential file. So to restore you need the full backup and only the last differential in tack to do a restore.
So a differential uses more disk space but will give you a greater change of recovery.