Since the humans adapted to living with undead neighbours in the past movie "Day of the dead", they built reinforced houses and cities in "Land of the dead". While a few wealthy individuals can live in luxury, most survivors are poor and entertain themselves with prostitution and zombiefights. To gather food and supplies, a taskforce named Dead reckoning visits neighbouring cities, kills the zombies living there (by distracting them with fireworks and shooting them - cowards! -.-#) and takes gas, food and everything that might be of use.
Main focus of this movies is an - more or less, don't expect Albert Einstein- intelligent zombie named Big Daddy. He learns and leads the other zombies to kill humans in the protected cities... I really recommend this movie because the zombies kind of develop a personality beyond muttering "Brains"!
Last, but not least in Romero's Zombie storyline is "Diary of the dead". This movie starts with an inception, a zombie movie being made in a zombie movie! The movie is done in retrospect, the zombie infection is not known and surprises the film team when they come to a hospital. They are constantly running away from zombies while the undead turn one after the other into brain-hungry zombies...
In comparison to "Land of the dead" this movie is a step back to the more traditional zombie movie with young people running away and screaming a lot. Nevertheless this move is worth watching, although it is kind of a letdown after Land of the dead with zombie evolution.