A backup is a temporary storage where you save your on-going work to keep a known-good version. It is useful to recover a file which has been damaged during modification or mistakenly erased. A CD-ROM or DVD is fine.

An archive is a long term read-only storage. It should contain files you won’t change, and be properly indexed to ease later search. No DVD here.

For those who are not convinced that writable DVDs are unreliable I have this short story: I recently found a disc which had stealthily fallen behind my desk out of sight and reach. The disk was partly out of its protective box and had been half-exposed to the sunlight for quite some time. The half which was still sheltered was dark blue, but the exposed half was light pink. The content was 0% recovered. You can try it for yourself by sticking a piece of opaque tape on an old DVD-R and exposing it to direct sunlight.