The Zombies
The origins of the outbreak are unknown to the characters and the readers. Since the storyline mostly follows Rick Grimes, who was comatose at the time the outbreak started, no information of how the outbreak began has been shown. There have been instances with the various introductions of new characters over the course of the series where their own perspectives and knowledge of the beginning of the crisis was revealed. Although few true facts have been confirmed, a recent character claims he knows the source.
In the first 12 issues of the series it was believed that zombies were only created from zombies biting a human. However, it has now been revealed and established that any human who dies with a brain intact will reanimate as a zombie, making all of the survivors already infected, and that zombie bites just kill the victim unless the area is cut off. Zombie body fluids getting on a human will not zombify them either, as Tyreese's zombie daughter's blood did not convert him, and the Governor kissing his zombie daughter didn't change him either.
While an unamputated bite kills, the body fluids are okay, as Tyreese got his zombified daughter's blood on his face and remained fine. Currently unknown is what happens when a human consumes zombie or infected flesh, as a group of cannibals have eaten (after cooking) Dale's infected flesh.
The zombies of Walking Dead resemble traditional horror film zombies in that they walk and crave flesh. The zombies are slow moving but tireless, requiring no source to function. Their most dangerous weapon is their bite, which will result in a slow death, but if the location of the bite is quickly cut away, death can be prevented.
In the first issue Rick discovered an emaciated zombie lying nearly immobile on the side of the road so it seems that the zombies can starve into immobility. The only way to "kill" a zombie is to destroy the brain, the most common methods being decapitation and gunshots.
The main characters have classified the zombies into two types: "roamers" and "lurkers". "Roamers" walk while seeking new victims. The roamers are attracted to noises, and tend to walk towards any new sounds, sometimes forgetting why they are walking towards it until a new sound occurs or victims are found. The "roamers" also follow groups of "roamers" (who are heading towards noises that they may have long forgotten) out of an inborn sense of curiosity, and when enough of them gather, it creates zombie packs and herds, with as few as several to as many as hundreds of them together. The "lurker" category simply remain motionless and "play dead" until a human gets close enough to get bitten or a zombie comes too close and the "lurker" "yells" at it.
It is implied that zombies may retain dim memories of how the human world functioned. While zombies are portrayed as unintelligent, they do seem to have some ability to be trained, and will generally become less aggressive to humans who they are kept around for a long time, as in the cases of "Governor" Phillip's daughter and Michonne's boyfriend and brother.
Zombies also steer clear of humans who are near other zombies, perhaps due to the smell of rotting flesh, such as when Rick and Glenn cover themselves in the gore of zombie bodies and manage to escape notice from other zombies until rain washes it off. Perhaps similarly, Michonne was able to travel by shackling two zombies, whose arms and lower jaws she had removed, to her.