DVD-R and DVD+R are different DVD recordable media formats. When they first came out, some companies sided with the DVD-R format, while others supported DVD+R (kind of like the current format war between Blu-ray and HD DVD). Eventually, companies just started making burners that could burn both formats. Nowadays you'll rarely find a DVD burner that records only one format...and most players will support both.
DVD-RW and DVD+RW - same thing, except the "RW" stands for "rewriteable" which means support for playback with the rewriteable media format. Again, most players nowadays support both + and -, as well as both "R" and "RW" formats.
