DVD-R: they can only be written once. Their main characteristic is that they are highly compatible with all DVD Players.
DVD-RW: they can be used to write and then erase data several times. This format can be played on most DVD Players.
DVD+RW: they can be used to write and erase data several times. Their main characteristic is that they write faster than DVD-R and are more flexible. In fact they allow real-time recording and are not linear, like a hard-disc. For this reason they are much more frequently used in IT systems, but are also less compatible with DVD players.
DVD+R: they can only be written once. They are primarily used to store data since their characteristics are similar to DVD+RW.
DVD-RAM: they can be used to write data several times, however they are not compatible with tabletop DVD players or common PC DVD players. This format is mainly used in IT systems.
