Superdrive/Pioneer DVR 105 problem

I've just started to do DVD backups using my iMAC G4 (screen on a
stick model)  with superdrive(Pioneer DVR 105) and find that the drive
doesn't recognize certain kinds of DVD+Rs (Maxell & Sony). It writes
just fine to Imation 8x DVD-Rs.
(1) Since I can't return the DVD+Rs, what do I need to do to use these
besides upgrading my superdrive hardware?
(2) What the heck is the difference between a DVD-R and a DVD+R?

Thanks,
Jim