That's a long one ac3cd
In short you have two different standards which are supposed to be DVD Player compatible (as long as your burning a DVD obviously)
DVD-R/RW and DVD+R/RW
DVD-R/RW are cheaper and more compatible with older DVD Players and can burn at 1x,2x and now 4x depending upon the drive and media.
DVD+R/RW drives have some extra features but the media costs more and DVDs burned with them won't play on as many DVD players.These drives are 1x,2.4x and now also 4x depending upon the drives and media used.
Both formats offer 4489MB of storage capacity (about 4.35Gig) otherwise known as single layer discs. The R/RW is the same as with CDs in that writable and rewritable versions of the discs are available. With either drive you could use the capacity to burn data discs as well as burning movies as either DVD format will hold about 7 CD's worth of information.
I have a DVD-R/RW and chose the DVD-R/RW over DVD+R/RW format because of compatibility and considerably lower running costs (ie media).