Different wavelength when recording. They are completely different recording formats. Authoring DVD are more toward professional uses. Suppose to be replacing master digital tape studios using when produce the dvd/vhs. Authoring DVD does have some portion that stores property of discs which is not there in DVD-R General purpse. I believe the DVD screener are produced with DVD authoring as well. Original DVD authoring is like 3.8GB, newer version is 4.7gb. Also, CSS writing is possible only on DVD authoring. (That area on general purpose is not writable.)
General purpose can be used for video as well. Both discs should be able to play on set-top.