A differential backup
is a backup
of all data
which have changed since the last full backup
. Note that this is different from an incremental backup
What is the advantage of a primarily differential over a primarily incremental backup strategy? Restoring from one full backup and one differential backup is rather faster than restoring from a large number of incrementals. While differentials take up more space on backup media, the speed with which you can restore service is often more important than the cost of media. However, the optimal backup strategy for your files depends on your time and funding constraints, and how often your files change.
Note also that some backup software supports a strategy which is a mixture of differential and incremental backups -- a leveled backup strategy.