What is an Incremental Backup?

A incremental backup is a backup of every file on a filesystem which has changed since the last backup.

An incremental backup is the fastest backup and requires the least storage space on the backup media. However, incremental backups also require the longest time and the most tapes to restore.
Incremental backups should be used only in environments where backup time or backup storage media are extremely constrained. For most environments, a weekly full backup and a daily diffrencial backup represent a better plan.
If you perform a full backup on Sunday along with incremental backups every night and the system crashes on Thursday, you will need to restore the full backup from Sunday along with the incremental backups from Monday, Tuesday, and Wednesday.
In contrast, if you perform a full backup on Sunday and a differential every night, when the system crashes on Thursday you will only need to restore the full backup from Sunday and the differential backup from Wednesday