A DVD-R is a write-once format: once you've burned the data onto that DVD platter, the disk is forever frozen with that information. Add the "W" to that, and you'll find that DVD-RW can be erased or rewritten up to a thousand times. Seems kinda weird, but if you can do so, DVD-RW obviously has significant advantages over DVD-R. DVD-RAM was even more flexible, however, since it let you erase and rewrite sections of an existing DVD, something that you cannot do with DVD-RW.

Moving to the plus side is where things get a bit confusing, because DVD+RW came before DVD+R. The plus formats have the same data storage capacity as the minus formats (4.7GB), but DVD+RW offers faster writing, better internal linking (a technical obscurity you don't have to worry about), and support for drag-and-drop desktop files, which makes it easy to compose the contents of a disk. DVD+R is a write-once format intended to be more compatible with more DVD players, though at this point it seems to be about even with DVD-R, which remains the most compatible computer-burned DVD format.