Why are zombies so hungry?
Zombies have no soul, so they have nothing to hold back their lust for human flesh (a desire which we all have, but is normally suppressed by our conscience).
Do they eat until they burst, or do they get full, digest, and poop it out?
Zombie 'digestion' is very efficient. Everything they eat is absorbed intact, directly into their bodies. Unfortunately however, they have trouble keeping their bodies together. Zombies have to keep eating because bits of them are constantly falling off!
Zombies get their energy from brains. To keep active they have to eat brains regularly (fresh Human brains are preferred because they are 'fully charged'). If you shoot a zombie in the head it looses all of its energy, falls down and cannot get up again (it's not 'dead', just inanimate).
Note: You have to destroy the entire brain, using eg. a shogun or high powered rifle (decapitation also works). Just injuring the brain is not enough - it has to splatter!
If blood cannot circulate, how are the muscles powered?
Zombie metabolism is anaerobic, which is much less efficient than aerobic respiration. That is why they move so slowly. Zombie muscle movement is controlled by animal magnetism. It works similarly to how electricity makes dead frogs legs twitch. This control is not centralized - each limb has 'a mind of its own' with limited coordination between different parts of the body. The energy source of the animal magnetism is the brain, but it has no control.
How can they tell humans from other zombies?
By our Aura. Zombies don't have souls so they don't have Auras. Zombies eyes are practically blind so they can't see normal light (unless it's very bright), but they 'see' in the spirit world (this is how they are able to get around at night without bumping into things).