(See also incremental backups
An incremental backup is a backup of all your data which have changed since the last incremental, differential, or full backup. Note that this is different from a differential backup, q.v.
The chief advantage of an incremental over differential backup strategy is that it consumes a lesser amount of backup media. Given that lots of people now find themselves faced with backing up a 20 gigabyte hard disk onto 4 gigabyte DDS tapes, the popularity of incrementals should be clear. 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.