Well the first thing to look at is weather the actual drive itself has the word DVD Rewritable actually printed on it, if not, then you can look at the device manager under CD/DVD drives and see what your drive is listed as, it will say there. And as for your question about the difference between DVD+R and DVD-R. They are simply different formats, you should only get the type that your drive supports since thats the only type that will work for you. The main difference between the two is that DVD+R is generally used for and is more compatable with home DVD players, where as you would GENERALLY use DVD-R for data storage or a movie that you will be accessing primarily from your computer.
--Chris