Is DVD Rom + or -???

[Joules]

Although I understand the difference between the two standards, there is still one thing that confuses me......

Which of the two formats is the standard DVD Rom?

I just bought a DVD+RW (the Ricoh one) and the only DVD Rom I have works fine (free with an older PC mag I bought).....Im presuming it also works with the older DVD- standard.....

If this standard works in both drives, why cant they make a DVD burner that burns DVD Rom, instead of having to choose plus or minus???!!!!!

Joules