I have been curious about this topic ever since the publication of Harry Potter and the Half-Blood Prince, and since Harry Potter and the Deathly Hallows is going to be published in five days time I think that the time is right to approach this topic seeing as J. K. Rowling isn't likely to address the issue in DH what with the other, more important, dangling plot threads to tie together. On re-reading the books in readiness for the publication of DH, I've been reminded of my curiousity and I'm determined to look for some answers!
At first glance Zombies and Inferi (both mentioned by name in the series) share so many similarities that it is possible they are two different names for the same things. A Zombie is traditionally an animated human body, devoid of a soul, although later ideas of Zombies have adopted the thought that the soul has left the body through death and the body has been reanimated. From what we hear of Inferi in HBP, they are pretty much the same thing, reanimated dead bodies under the control of the wizard who reanimated them.
It could be possible that Zombies and Inferi are the same creature under a different name, Dodos are known as Diricawls in the wizarding world after all. However, Snape, who has given thorough accounts of other dark creatures before, failed to tell the class that Inferi are also known as Zombies and in the interest of being thorough with the class about this addition to Lord Voldemort's forces, Snape surely would have told the class that both names for the 'living dead' are at least acceptable academically.
Then we have the case of Seamus Finnegan. As the son of a witch mother and Muggle father who was brought up in the wizarding world, Seamus eagerly accepted Professor Quirrell's story that he fought an African Zombie and was interested in how Quirrell defeated it (PS/SS, ch 17), yet in HBP (ch 21) Seamus asks Snape how to tell the difference between a ghost and an Inferius. This suggests to me that Zombies and Inferi are clearly different creatures as Seamus was aware of what Zombies were in his first year, yet cannot tell the difference between an Inferius and a ghost in his sixth. However, when Harry describes the difference ("Yeah, ghosts are transparent but Inferi are dead bodies, aren't they? So they'd be solid -" pg 431), it appears that Inferi and Zombies are extremely similar after all.
Therefore, the difference between the two creatures must be semantic. As traditional voudoun lore states that a Zombie is simply a living human devoid of a soul, perhaps Inferi are the opposite; human bodies raised from the dead, yet soulless and missing the personality they had as living humans (Dumbledore has said that no spell can bring the dead back to life in the way in which we know them now). Whilst a Kiss from a Dementor can remove the soul from a living human, leaving the person a shell but still alive (PA, ch 12), Snape tells us that an Inferius is created when a dark wizard reanimates a dead body using certain spells (HBP, ch 21, pg 431). Perhaps, then, Zombies are those wizards who have been on the receiving end of a Dementor's Kiss, whilst Inferi are strictly dead humans who have been reanimated by dark spells.
Lord Voldemort has used Inferi in the past to terrorise his enemies and has a lake of Inferi guarding the location of one of his Horcruxes, Slytherin's locket. Inferi are under the control of the wizard who reanimated them. In contrast, once a person has received a Kiss, they are soulless and alive, yet 'worse than dead'. I believe this statement to mean that once someone has been Kissed, they are unable to care for themselves and must rely on others to care for them for as long as they continue to live. The Wikipedia entry on Dementors likens the effect of the Kiss on a human to a Persistent Vegetative State, where the patient is unmistakeably alive, yet in the main unresponsive to outside stimuli, although some PVS patients react to pain stimuli and, either in response to stimuli or completely unprovoked, exhibit semi-conscious behaviour such as crying, screaming or grinding teeth and could experience sleep-wake cycles, or be in a state of wakefulness with their eyes open persistently.
In contrast, the Inferi Harry and Dumbledore fought at the end of HBP responded only to heat; Dumbledore conjured a fire to protect Harry and himself as the Inferi prefer dark, damp places. Harry used, amongst others, the Sectumsempra curse, yet it had little effect as the Inferi were simply reanimated long dead bodies with no blood left to spill. Had he used Sectumsempra on a Zombie (as in, someone who has been Kissed by a Dementor) Harry would not likely have caused the person to regain consciousness, but blood would have been spilt and he would have caused the Zombie pain.
If my theory is correct, Barty Crouch Jnr is now a Zombie and there must somewhere in St Mungo's Hospital for Magical Maladies and Injuries, presumably a closed ward on the First Floor (Creature-Induced Injuries), where these Zombies are being cared for by healers. There is no cure for a Dementor's Kiss, yet as the Kiss does not kill, these recipients are in need of constant care and attention particularly if they were administered the Kiss accidentally rather than on Ministry instruction. In contrast, Inferi are at the mercy of the wizard who reanimated them. They need no food or rest because they are dead, they could be put to constant work and their toil wouldn't affect them in the slightest. Inferi are still capable of movement and sense, whereas a Zombie is not. A Zombie cannot move from their bed and can perform only small movements such as blinking and perhaps slight movement of fingers and hands.
We are likely to see more Inferi attacks in DH seeing as HBP ended with an overt introduction to the concept that Lord Voldemort has used Inferi in the past and is not averse to using them in the present. We may even see a connection to those who were killed by Avada Kedavra making the best kind of Inferi as the bodies are left unmarked and undamaged, compared with other deaths such as old age and disease. Lord Voldemort's creation of Inferi may even be related to specific deaths...a wizard may only be able to reanimate the bodies of those he has killed.
However, J. K. Rowling probably intended Inferi and Zombies to be the same thing and didn't realise that she had already mentioned Zombies earlier on in the series. Inferi simply means 'the dead' in Latin and Zombies are colloquially known as 'the dead' in many Zombie films, comics and books. Even if her intention was to have Inferi and Zombies be one and the same, the fact that both are mentioned in the series and no connection is made between them (even Muggle-born Hermione and Muggle-raised Harry make no connection between the Inferi and popular culture Zombies), indicates that the two creatures are unconnected. It would be nice to hear a definitive line on the topic though, because the implication is quite serious for those who have been Kissed and for those who die of Avada Kedavra. It could even mean the return of Dumbledore under the control of Snape if my theory over the creation of Inferi is correct.
I really can't wait to see what DH is going to bring to the table!