This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
Simply put, they aren't.
There is no such thing as CD+R or CD+RW. There are a number of different DVD
formats, including DVD+R and DVD+RW, but so far CDs only have -R and -RW.
CD formats with a '+' in them (except for CD+G, which only defines the
subcode channels of an audio CD) are usually typographical errors.