Differential Backup / Image
A differential backup checks what has been changed since the last full backup and only saves those changes. When restoring a differential backup both differential image and full backup image must be available on same folder on backup storage drive.
Because only changes since last full backup will be saved, differential images can be quite small compared to full backup, of course depending how big changes user has made. The later a differential backup is created, the more changes have been made, the bigger the differential image will be.Incremental Backup / Image
(Not available in Free edition!
An incremental backup checks what has been changed since the last backup regardless if the last backup was full, differential or incremental and only saves those changes. When restoring an incremental backup all differential and incremental images and full backup image must be available on same folder on backup storage drive.
Incremental images can be really small. An example: if you take daily incremental backups and today you just browsed the web and got a few emails but did not download or install anything, today's incremental backup is only a few kilobytes, a megabyte or two max.