im not sure entirely, but the *very* basic way i had it explained to me was that when dvd was starting up there were two major companies researching it. they both came up with there own way of essentially the same thing.
so one is plus and one is minus
its not nesscessarily bad that your dvd plays only minus, if you write dvds just make sure they are in minus format else your dvd player wont read them
sorry ic ant be more help on technical differences between plus and minus, howstuffworks.com might be a good place to try for the tech stuff