[simcop2387]

Basically like others said, there's some kind of physical separation. That way if the machine were to explode or you loose too many drives in the array you still have a copy of them. RAID protects you from a hardware failure (to some extent), a backup protects you from hardware failure, software failure, and human failure (or it should anyway). The idea is that if the cannonical version of things gets destroyed somehow the backup is available to rebuild, so it can't be alive in the same machine as the cannonical one (except maybe when doing the backup itself).

The other "rule" that many people follow is 2 is 1 and 1 is none. The idea is that anything that isn't backed up isn't protected and can't be relied upon to exist.

A cloud service like backblaze, google drive, amazon cloud drive, etc. is a good secondary backup for a lot of people even if it'll take you a month or two to get your data there to begin with.