DVD-R/RW and DVD+R/RW

What is the difference, and why would a HP DVD Writer not be DVD-R compatible?

DVD+R are more expensive?, is that true?

any help on this matter would help basically i have a PC that has a DVD burner and through Nero 6, it can not burn DVDs, however found out, that it doesnt support DVD-R/RW???, havent got a clue why?,
model is HP Writer 300n

Thanks for any help, cheers