There is +, - and RAM...Then within those theres R, RW and Authoring...or something like that. Basically, as far as I remember, most standalone DVD players prefer - discs, with some newer ones taking - and +.
- is supported by the DVD Forum (the people who created DVD technology), whereas + is currently not. However, many big makers (Song, Pioneer etc) still make + drives without giving a damn about the Forum (as they have no power over what the world does w.r.t. DVDs).
RAM, i think, are the ones that are created professionally for proper video DVDs etc, and the equipment needed costs muchos buckos.
Both formats currently have the same market share, yet - (as I said) are better for DVD players (standalone).
Basically, +/- is better than either on its own as you are futureproof in regard to whether one day, people may suddenly and irrationally decide to discontinue the + variety of DVDs when thats all you have! Also, it means you dont have to take that little bit of extra care when buying your DVD-R/RW discs in the shops to make sure you pick up + instead of - (and vice-versa).
Search it on Google.com, theres many guides on the differences.
Although it says +/- on the option for the US site, click the 'More Info' button and it lists it there as a +/- for the name of the hardware at the top, but in the boxes it still says DVD+RW...no -.
And as usual, on the UK site everything says +...should be expecting the +/- in a couple of years lol :-P
Hope this helps, feel free to correct me as I havent looked this up in a while!