The WGA is actually two guilds East and West. The east guild has affiliation with AFL-CIO
The guilds represent writers of TV, Film and Radio. Adverstising writers, at least the ones I know are not union members.
Of course not all writers are covered. Oddly enough in the old days shows like "Real People," WERE covered with agreements. These shows, to me at least, along with "That's Incredible" etc were just as much reality shows as "American Idol" and the lot which aren't covered.
Also with the HUGE exceptions of "The Simpsons," "Family Guy," "King of the Hill" and "American Dad" TV animation isn't covered either. Most animated films aren't covered either. (Some are if they negotiated seperately)