Both formats have dual layer options (DVD-R DL and DVD+R DL).

The differences are technical. The + format is less susceptible to burning errors for a number of reasons, so it was also able to reach faster burning speeds.

Modern burners and players can record to and playback both + and - media, so to end users their differences are effectively nullified. But generally if you look at specs closely, you'll notice that many dual-format burners have slightly faster + burning speeds than - burning speeds.