Whats the difference between DVD - R and DVD + R ( for recordable dvd players)?

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. 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.

Not much really. Most modern DVD players and burners can use either type. Older models need to version they were built for. As for the disks, the storage capacity and video/audio playback is the same quality so there is really no difference to the end user.

The only difference is compatibility. The size, storage capacity and cost are exactly the same. DVD-R tends to be compatible with more DVD players than DVD+R. If you plan on copying movies, I would suggest using DVD-R media.