DVD-RW and DVD+RW are both rewritable DVDs, which when recorded are suppose to be compatible with existing DVD players and DVD-ROM drives depending on the content written to them. DVD-RAM is another rewritable DVD format, however as it is incompatible with the existing DVD specification, this means that only DVD-RAM compatible players can playback this disc type.
However, when it comes to DVD recorders and PC DVD-writers, each format uses a different method of writing, which means that the disc type must be compatible with what the recorder / writer is capable of writing to. All recent PC DVD writers that I’m aware of will write to all DVD formats apart from some that don’t support DVD-RAM. However, a lot of standalone DVD recorders still only support one disc type or a combination of them, like your DVD recorder. Each disc format also offers its advantages, drawbacks and features over another when it comes to writing. Page 4 of your user manual lists the main features of each disc format as well as what features can and cannot be performed with each disc format.
The following (fairly old) article covers some in-depth technical caparison between DVD+RW and DVD-RW (mainly why DVD+R/RW is suppose to be supperior to DVD-R/RW):