newbie question about dvd-r/cd-r

Hello!

I have a newbie question about dvd-r's and cd-r's. I've noticed some
computers are coming out with both a dvd-r and cd-r. Others only have
a dvd-r.

Is there an advantage of having both a dvd-r and a cd-r? Or is it a
redundant feature?

One last question, can anyone please explain this:
dvd+r/+rw/-r/-rw/cd-rw drive

Thank you.
al