Differential Backups

Differential backups are similar to incremental backups in that both backup only modified files. However, differential backups are cumulative -- in other words, with a differential backup, once a file has been modified it continues to be included in all subsequent differential backups (until the next, full backup, of course).
This means that each differential backup contains all the files modified since the last full backup, making it possible to perform a complete restoration with only the last full backup and the last differential backup.
Like the backup strategy used with incremental backups, differential backups normally follow the same approach: a single periodic full backup followed by more frequent differential backups.
The effect of using differential backups in this way is that the differential backups tend to grow a bit over time (assuming different files are modified over the time between full backups). This places differential backups somewhere between incremental backups and full backups in terms of backup media utilization and backup speed, while often providing faster single-file and complete restorations (due to fewer backups to search/restore).
Given these characteristics, differential backups are worth careful consideration.