Zombies are always after human beings. How do Zombies differentiate between a zombie and a human being? Why don't zombies eat each other?
It appears to be related to their sense of smell.
In "Guts" (season 1, episode 2), Rick and the group cover themselves in the guts of walkers in order to disguise their scent, and it seems to work. They only get attacked when rain washes away their disguise.
It's reasonable to assume walkers won't attack any moving creatures that smell like them.
Without trying to sound like a smartass, zombies differentiate between humans and other zombies more or less the same way you do. It is a combination of factors, rather than a single difference.
Zombies look... well... like zombies. People look like people. This one is pretty self explanatory.
Although I have frequently pondered the question of how bad the people in The Walking Dead must smell, having had few opportunities to bathe properly since the world fell apart, it still seems clear that the people smell different from the zombies. Sweaty, unbathed Rick probably stinks like rancid body odor, but the zombies smell like the rotting corpses that they are.
In fact, the zombies stink so bad that in season 5, we saw Glenn, Maggie, Abe, Rosita, Eugene, and Tara almost reeling from the stench of a herd of zombies far off in the distance. And in season 1, episode 2, Andrea explicitly said that zombies can smell people. That's why Rick and Glenn covered themselves with zombie guts before walking into the herd outside the department store.
Humans make fairly unique noises, none of which can be made by zombies. We talk, we laugh, we cry, we scream, we cough, we bang around and build stuff, etc. Zombies are attracted to all kinds of sounds, but they are particularly attracted to the kinds of sounds that humans make, or that are associated with humans (including things like car engines).
People move in different ways than zombies. Whereas they stagger around in a lurching, uncoordinated manner, we usually move with purpose, heading in a specific direction, and in a relatively coordinated manner. We have a good sense of balance, we move relatively quickly compared to zombies, and we show far more deliberation in our movements - we are trying to get somewhere, not just wandering around with no particular destination in mind.
We also make other kinds of movements: we fiddle with things in our pockets, we stop to get a drink, we carry guns and supplies on our backs, we open and close doors and windows, and so on. All of these movements and motions are quite different from those of the zombies.
Zombies don't act the way we do. If there is no prey in the area, they just hang around waiting for some to show up. Sometimes they stand still or even sit down; sometimes they stagger around, more or less in circles. When they do the latter (staggering around in circles), they do so in a uniquely zombie-like way: they make no attempt to avoid walking into each other. They don't even try to avoid walking into stationary objects. They bump into things, and into each other, and don't even seem to notice it.
Humans, on the other hand, don't do any of these things. We generally don't stand around doing nothing, or lurch unsteadily in circles, or walk directly into other objects. We are hardwired to avoid walking into things. We don't like bumping into stuff, so we usually don't do it.
Zombies don't respond to anything except potential prey, such as people or animals. In fact, it seems that they actually don't even notice anything other than potential prey. If they don't hear or see any signs of nearby food sources, they don't look around much. They just stand there and wait, or stagger around and wait. They take no interest in each other or anything else that isn't food.
Humans are nothing like this. We are constantly looking at everything around us, and in the world of TWD, we have good reason to look around - we need to find food, shelter, water, supplies, etc. We pay attention to our surroundings and everything in them. When we encounter an obstacle, we stop and think about how to get around it, whereas a zombie would simply walk directly into the obstacle.
Following the Herd
Sometimes, zombies attack people they can't actually see, hear, or smell, simply because other zombies are already trying to get at the people. If a herd of zombies is piling up outside a house or whatever, presumably because they know that there are people inside, it sometimes happens that other zombies, who just happen t be walking by, will join the herd, despite the fact that they themselves have not heard, seen, or smelled the people inside. They just seem to know that if other zombies are interested in something, it is probably food of some sort.
For more information about the herd dynamic, see my answer to this question.
The most obvious example of a special case, in which a person avoids attracting the attention of zombies, is Michonne. This woman is a genius. She realized that if she was accompanied by zombies everywhere she went, other zombies would assume she, too, was a zombie.
But things are never quite that simple. She had to figure out how to prevent her "pets" from giving her away by constantly reaching out to grab her, which would obviously attract the attention of the other zombies in the area. Her solution was brilliant in its simplicity: she hacked off their arms. To prevent the pets from trying to bite her, she also removed their lower jaws.
It appears that Michonne's pets break one of the central rules of zombie-hood: they learned that they would never be able to bite her, so they stopped trying to do so. According to the rules of Brooks zombies, Romero zombies, and every other TWD zombie, zombies can't learn anything, ever. But somehow, the pets figured out that they couldn't get Michonne, so they stopped trying.
Michonne walks slowly, her pets follow closely behind her, and all the other zombies in the area just assume she is a zombie herself. She is exploiting several of the factors that zombies use to distinguish between other zombies and living people: her motions are zombie-like, the scent of the pets covers up her human scent, and the pets' inability to reach out for her avoids the possibility of the other zombies being alerted to her presence.
Seems to be a few factors:
- S01E2; walker gore worked until the rain set it (at which point they started smelling humans)
Reaction (human/herd based)
- Like any other predator, they will chase what flees from them. While the pet walkers would have smelt somewhat, other walkers noting no reaction from them would have been important, probably even more so given their herd mentality. A different reaction sparks mimicing, or suspicion, depending on what it is.
- Walkers have come to assicoate sound with the presence of prey and will follow it for a considerable distance (Hershel's Farm and the helicopter).