This is because when DVD writers first came out companies such as Pioneer and Toshiba created a write-once format which they named DVD-R and a re-writeable format which they called DVD-RW. Other companies joined the DVD-R format and the DVD Forum was founded. The DVD Forum is now an international association of hardware manufacturers, software firms and other users of Digital Versatile Discs. Dealing only with DVD-R DVD-RW and DVD-RAM. About 220 companies participate in the DVD Forum.
Later on other companies such as Hewlett-Packard, Philips and Sony thought that they could do better than the DVD Forum so they designed their own DVD format and named it DVD+R and DVD+RW. They hold the same amount of data but claim to be more compatible with set-top DVD Players.
Both DVD "minus" and DVD "plus" will have support for years to come and you can use both formats in modern DVD-ROM and DVD video players. It really doesn't matter which one you decide to go with. The latest DVD Re-Writers are now offering support for both DVD-R/RW and DVD+R/RW.
