If you're only using it for data backup then any -R or -RW format should do. However, if you want garunteed playback in set top DVD players then +R or +RW (if you can find one =) ) is probably what you want. Here's how it breaks down:
DVD Audio
Much like CD audio but with capabilities of storing higher sampling frequencies (96khz) which includes support for Dolby Digital AC-3 and surround sound and can also include text data for track titles and some other stuff as well as copy protection and some anti-piracy stuff. Not likely to replace CD's but hey, someone had to try it right.
DVD Video
This is what most set top DVD players use. The total capacity is 17 gigabytes if two layers on both sides of the disk are used.
DVD-ROM
Your average run of the mill DVD player for the computer. Includes DVD-VIDEO and DVD-AUDIO playing capabilities plus a bunch of stuff for storing files and the like.
DVD-RAM
DVD-RAM is kinda like a virtual hard disk with a random read-write access. Capacity of 4.7GB per side and is completely re-writable. However, DVD-RAM disks can not be played in existing DVD players and DVD-ROM drives. DVD-RAM disks require a DVD-RAM drive for playback.
DVD-R (-R)
Capacity of 4.7GB per side, just like DVD-ROM but it's a write once only deal. However, DVD-R disks can be played in most DVD players and DVD-ROM drives.
DVD-RW (-RW)
Just like -R but it's re-writable. Most disks can be re-written up to 1000 times, give or take depending on the quality of the disk. Can also be played in most DVD players and DVD-ROM drives.
DVD+RW (+RW)
Another standard developed by a whole slog of big name consumer electronics companies. DVD+RW has a read-write capacity of 4.7GB per side and is the only re-writable format that provides full compatibility with existing DVD-Video players and DVD-ROM drives. It can also read and write to CD-Rs and CD-RWs. A sub version of +RW has also been created called +R which, like ?R, is write once only.
Different companies have drives out with different formats (some with both), Sony has a new drive out that reads and writes both + and - formats. Although, if you ask me both + and - formats on the one drive is a waste of time, all you really need is +. The current problem is the lack of drives that actually support +R and +RW, obviously +R and +RW will cost more, -R and -RW being more common, sill have their uses. Some DVD-R and DVD-RW drives also support DVD-RAM but if you ask me, DVD-RAM is a dead end standard.
I say make you decision on the need you have for the drive and how fat you wallet is. If you can't afford (or can't find) a + drive then a - drive would probably be just as good.
Edit: As of yet there is no commercially available driver or media capable of burning dual layer DVD disks and I doubt one will ever be released. I think DVD+RW will be the nail in the coffin for the DVD formats simply because it has the backing of so many big name companies like Sony, HP, Dell, Philips, Richo etc... The only other likely place a new DVD format will come from is Pioneer who developed the -R and -RW standards, but I seriously doubt that'll happen.
Anyways, I hope that helps.
[This message has been edited by Dracusis (edited 05-27-2003).]