If possible, buy a DVD burner that can burn to BOTH DVD-R and DVD+R recordable media. If your DVD writer only supports one type of DVD media, you will need to know that before you buy DVD recordable media.
If you already own a DVD burner and a DVD player (like most of us), find out what your DVD player can play, then use that type of DVD recordable media. Most newer DVD burners support both formats so focus on what your DVD player can handle.
So what's the difference you ask? DVD-R and DVD+R are two competing technologies that use different formats.
DVD-R/RW was developed by Pioneer. Based on CD-RW technology, it uses a similar pitch of the helix, mark length of the 'burn' for data, and rotation control.The DVD-R format has been standardized in ECMA-279 by the Forum, but this is a private standard, not an 'industry' ISO standard like the CD-R/RW Red Book or Orange Book standard.
DVD+R/RW is also based on CD-RW technology. DVD+R/RW is supported by Sony, Philips, HP, Dell, Ricoh,Yamaha, and others, and has recently been endorsed by Microsoft. DVD+R/RW is not supported by the DVD Forum, but the Forum has no power to set industry standards, so it becomes a market-driven issue.