When a new/blank DVD-RW is initialised, it will give a choice of quick and full initialisation. While both quick and full initialisation gives pretty much the same recording and editing features, full initialised DVD-RW discs are always ready for playback in other players and cannot be finalised.
However, a quick initialised DVD-RW must be finalised before it can be played in another player. Unfortunately, once a quick initialised DVD-RW is finalised, it must be erased before it can be used again for recording. Just bewarned that fully initialising a DVD-RW takes about 15 minutes for a 4x disc and about 30 minutes for a 2x disc. DVD+RW's can only be quick initialised, however as mfdemicco mentions, these are always ready for playback in other players.