For those not caught up on their zombie lore (KPMD, I'm looking at you), there are a couple of great resources on the internet:
-Homepage of the Dead focuses mainly on Romero's films.
-All Things Zombie covers, well, all things zombie.
And then there's this:
Chart created by Groucho at DVDTalk
Between the chart and those two sites (and their myriad of links elsewhere), you should have all of the information necessary to carry on an intelligent zombie conversation at your next cocktail party.