I'd always wondered about what is the difference between DVD-R ("DVD minus R") and DVD+R ("DVD plus R") discs. Virtually every DVD burner supports both formats (including DVD-RW and DVD+RW), and the capacities of the discs are the same. Both exist in double-layer ("DL") versions, as well.
But the fella over at this blog uncovered a rather heinous difference: DVD-R discs include a small, pre-recorded ring used for data encryption. No, not in a good way: These discs take longer to copy and, in some cases, can't be copied at all due to the pre-recorded crypto ring, which can't be overwritten. (The upshot is that this can prevent "disc-at-once" copying, or a complete bit-for-bit copy, because a portion of the disc is un-writeable.)
This is complicated stuff (if you don't believe me, try decoding the graphics at the bottom of this page), and I'd wager it's that confusion that companies and groups like the DVD Forum are relying on. You'll buy DVD-R because it has an official DVD logo on it (pictured).
In the absence of any compelling reason not to use DVD+R discs instead of DVD-R discs, this writer advises you to go with the former. But then again, maybe this issue is getting blown out of proportion.