There are three traditional backup types--full, cumulative and differential.
Both of the following types are considered Incremental backups, they can be used independently of each other or in conjunction with each other but always relate back to a FULL backup.
If you site is not too large, then FULL backups are the way to go, once a week at least. If your content changes quite regularly or more importantly can not be recreated or is too costly to recreate, once a night or more may be more effective.
If time, server resources, or the rate of data change is too high to successfully obtain a FULL backup every night then the incremental backups are needed.
If you choose to use a cumulative backup following a weekly full, the backups each night will run quicker than a full backup, however as the week progresses, each nightly cumulative backup will increase in size and time, due to not only backing up the changes since last night's backup, but it also backing up all changes each night and previous nights since the last full backup was made. The benefit of this type of backup, in conjunction with full backups is the speed of restoration. To restore, you now only need to recover the most recent full and cumulative backups to fully recover all information.
If time or server resources are paramount or data change overwhelms cumulative backups, turn to incremental backups, this style of backup when used in conjunction with a full backup will provide a very similar level of protection, but restoration will be slower. Incremental backups will only backup changed data since the last backup of any type, not since the last full backup, as with a cumulative backup. Thus, when restoring data, you will need to recover the full backup, then each incremental backup in turn (oldest first) in order to fully recover all information. This method also has the drawback of recovering any legitimately deleted files, potentially "over-filling" the file-system.
For the average Web site, a daily or weekly full backup of both site files and database records is normally more than enough. Keeping a number of backups for a period of time is always a good plan, maybe keep each weekly backup for one month. This allows you to recover an old site in the case of emergencies or if for some reason you have local backup file corruption.
There are many PHP and Perl scripts on the Web that can be automated through CRONTAB and can either email (if small enough) or FTP the backup files to an off- or cross- server location. Remember that to some degree with Joomla! you already have an instant backup of the core files, if you haven't modified core, the Joomla! distribution files can be easily restored. Then you need only worry about backing up changed files and the database.