Regardless of + or - type, there is another variable.
DVD5 is a standard disc. It has a nice picture of what's on the disc on one side, and the actual DVD clever stuff on the other side. There is only one layer of material to record information on.
DVD9 is similar to DVD5, except it's like taking two regular discs (DVD5), and sandwiching them together so the two picture sides touch. That way, you get two shiny sides to each disc, and once at the end of side A, you have to take the disc out and swap it for side B. Still just one layer of information stored on each side of the disc.
However, there exist two other types of disc. These types are produced by sandwiching two information layers together. These are called DVD10 and DVD18 respectively (just the previous numbers x2!).
DVD10 is like taking two regular (DVD5) discs, and sandwiching them together but so that picture side of one touches the shiny special side of the other. So you still have a shiny side and a picture side to the disc, but inside there are two information layers -- meaning that you can store twice as much stuff.
This is what most films do that you buy commerically -- the feature film plus extra features are too big to fit on a standard DVD5 disc. Because the laser in the machine has to jump from one level to the other, you often get a slight pause in the action when watching your film.
Finally, DVD18 is the result of taking two of these DVD10 discs (picture side, shiny side with two layers of information), and sandwiching them together such that you can only see the shiny sides. So it's just like a DVD9 as described earlier, but with two layers of information on each side of the disc.
Why is all this important?
Often you will see advertised (for more money) dual layer discs -- with two layers of information -- DVD9. There have been recordable devices for PCs that can use these