The difference between DVD-R and DVD+R is how tracking/timing metadata is encoded on the disc. As manufactured, the disc isn't entirely "blank," it's got tracks for the laser to follow already on it, which include timing marks to help the drive track and index the data. -R and +R discs use different techniques for doing this, almost entirely because Sony just can't handle anything that isn't their own in-house proprietary shit.

This didn't effect playback. Once the disc was burned, that metadata was still there but an ordinary DVD-ROM drive (or DVD movie player) wouldn't even look for it, for playback, it looks like any other disc. It was only an issue for recording, and as a user you had to know which your DVD burner was capable of using so you could buy the correct discs. Eventually DVD burners were built to handle both, and today unless you have some vintage equipment it's a non-issue.