Zombies

Zombies are the main non-human Antagonists of The Walking Dead. The zombies are the classic slow walking Romero style zombies. The reason for why they turned isn't explained yet, however it is believed that they are all infected and the bites from zombies just kill them, because they are able to reanimate even if not bitten.

Behavior
Zombies are separated by the survivors in two groups Roamers and Lurkers.

Roamers
Roamers are known to "roam" around looking for food. They are the most commonly encountered kind of zombies.

Lurkers
They are the zombies that just sit around playing "dead" till someone reaches them and they bite them. This ones can be the real treat because they can pass as dead zombies. One of them was responsible for Allen's death and Dale loosing a leg.

Biters
Alice, Dr. Stevens' assistant in Woodbury mentioned that her original group of survivors refered to the zombies as "biters," because while some do lurk and some may roam they all will bite and to classify them into separate groups is a silly practice.

Herd
As described by Dr. Euegene Porter, it is when zombies act with a mob mentality. One zombie might bang into a house causing another zombie to think there is something inside. Yet another zombie will notice and soon a chain reaction occurs where a large group of zombies have gathered. This large group of zombies is known as a herd.

Weakness
Zombies are only vulnerable to massive brain damage, (i.e. Shooting in the head). Zombies that are decapitated will keep their head alive until destroyed. The first zombie encountered by Rick Grimes outside of the hospital appeared to be immobilized after some kind of decay. Zombies are shown to be frozen, one almost bit Glenn.

Strengths
Zombies are stronger than humans because they do not need blood or food to keep working, this allows them to slowly follow the survivors around in perpetuity.