read here, for more than you ever wanted to know :
http://www.proactionmedia.com/dvd_media_formats.htm
the only really important thing, before buying media, is to know what your CD/DVD Player/Burner is/are capable of writing to or reading
it seems that older players are the ones that are limited with ref to types of media - I believe most new(er) ones burn/play most all media
if you're in the market for new hardware, get one that has the capability to "use" all formats - there will be a list - usually on the outside of the box - that will show what formats the unit is compatible with - or, for existing Players/Burners, your owner's manual will list the formats
I burn DVDs in both +R and -R formats but all my equipment can read both
I use an RW format ONLY when I know I'll be re-writing or adding-to what I've got on the DVD/CD (for instance, a DVD upon which I keep adding digital pictures, as I take them)
RWs are more expensive than +/-Rs