After George A. Romero and John A. Russo made Night of the Living Dead, they parted ways. Each wanted to add more to the NotLD universe, so they agreed further sequels would exist independently of each other, creating two fictional universes. Romero's would follow the 'of the Dead' naming convention, while Russo's could use 'of the Living Dead.'
The Living Dead Series
Russo and Dan O'Bannon wanted separate to separate their story from Romero's already-established take (Dawn of the Dead had already come out seven years earlier, and Day of the Dead was coming out the same year). Their treatment injected more comedy and fun than Romero's apocalyptic renditions. The result, Return of the Living Dead, is one of my favorite horror movies of all time.
Return of the Living Dead (1985) Trailer
Man I love that flick. To me it's just a perfect 80's horror movie. The sequel, Return of the Living Dead 2, wasn't as good, but it's worth checking out. Unfortunately the story is not really a direct sequel, but a different zombie story in another place with different characters (even two of the main actors from the first movie play new roles - kinda bizarre).
Return of the Living Dead 2 (1988) Trailer
A few more sequels in the Living Dead story followed, but they have very little to do with the first two movies other than the name and having very little intentional comedy (although I found myself laughing at them several times). The third one, Return of the Living Dead 3, is at least watchable, but it's disappointing that it wasn't better as it was directed by Brian Yunza (Re-Animator).
The Dead Series
Romero on the other hand, in my opinion, continues to this day to put out quality zombie movies. Instead of ignoring the events of earlier movies, as the Living Dead series does, each Romero movie advances the overall story in some way, despite being accounts of different people and events. The entire series has a sense of continuity, each entry adding more layers and detail, building towards a cohesive overall story.
Dawn of the Dead (or Zombi internationally) essentially picks up the story where NotLD left off - the zombie invasion is well underway. Eventually the protagonists barricade themselves in a shopping mall, creating an awesome survival scenario - the mall gives plenty of access to supplies, but also is a giant magnet for the undead. DotD had one of the greatest tag lines in horror movies history, "When Hell is full, the Dead Will Walk the Earth," giving a possible answer to on-going question "Why is this nightmare happening?"
Dawn of the Dead (1978) Trailer
By the time Day of the Dead takes place, the zombies have begun winning the war - they now outnumber the humans. Day follows the events inside an underground bunker, where scientists struggle to find a solution to the zombie problems. However, their military counterparts are growing frustrated with their lack of result, leading to building tension and hostility between the humans.
Day of the Dead (1985) Trailer
That wasn't very satisfying was it? Here, check out the Japanese trailer, which is packed chock-full of zombie awesomeness!
Day of the Dead (1985) Japanese Trailer
All of Romero's zombie films up to this point had been relatively low budget productions with little-known actors. With Land of the Dead Romero was finally given a half decent budget to work with ($16 mil) and some recognizable actors (Dennis Hopper, John Leguizamo). As Guillermo del Toro said, "Finally someone was smart enough to realize that it was about time, and gave George the tools. It should be a cause of celebration amongst all of us that Michelangelo has started another ceiling."
Land takes place even farther into the future. Humanity has built cities walled off from the undead, where society's elite can be safe. Fighting with the undead continues for the less fortunate of society, outside the walls. However, resentment grows among the downtrodden, and the zombie themselves seem to be developing more intelligence, becoming more organized...
Land of the Dead (2005) Trailer
Romero took Diary of the Dead in a different but familiar direction than his previous movies, taking us back to the events of Night of the Living Dead, but setting them in modern society. It's the first night everything changes, but this time in a world of cellphones, the internet, and digital media. I thought it was a great choice to do this, taking the Dead series in a new direction while returning to the story to its' roots.
Diary of the Dead (2007) Trailer
Romero is currently shooting another Dead zombie movie (woohoo!), currently with the working title Island of the Dead.