Well, I'm no expert, but I think spells are temporary. I mean, Avada Kedavra, for example, doesn't hang around for weeks. By the same token, the Fidelius Charm doesn't wear out after a few seconds. So, spells have an immediate, permanent effect but not a LASTING effect, if you take my meaning. Charms have a permanent, lasting effect. Does that make sense?
I think a charm is an enchantment on an object (Wingardium Leviosa, Reducto, Accio), while a spell is anything else, such as Lumos, Avada Kedavra, or Crucio that can produce something or an attack/defense magic.
I just always figured spells covered the broad range of every type of wand-word magic, and charms were a specific type of spell that had to do with using a spell to enchant an object to give it new properties, or something. Here's what Lexicon says on the issue:
Charms are a type of magic spell concerned with enchanting an object to behave in a way that isn't normal for that object. For example, the Summoning Charm brings an object to the caster. Charms is also something of a catch-all for spells that aren't Transfiguration (spells that change the inherent nature of an object). In some sense, if a spell isn't Transfiguration, it's probably a Charm.
A Charm might cause something to flash different colors. It might cause an object to levitate or even fly through the air. Charms can make a person laugh or dance or even create a bubble of breathable air around a person's head. In all of these cases, the object or the person doesn't really change, they just do something unexpected.
Some Charms can be extremely powerful. The Fidelius Charm, for example, can completely hide a person or a place in such a way that no one can find them unless they are given the location by a Secret Keeper. Memory Charms can be so strong that they completely remove a person's memory or even damage his or her mind permanently.
Charm spells are in some ways the opposite of Curses: many Charms seem to have an inherent positive tone (e.g. Tickling Charm) while Curses have an inherent negative one. This is not to say that Charms are weaker magic; a well-chosen Charm is a powerful magical tool against Curses, Jinxes, and Hexes. Professor Flitwick, the Hogwarts Charms teacher, was rumored to have once been a dueling champion, after all (CS11).
I'm also confused about the difference between Charms amd Transfiguration; they were changing animate and inanimate objects in both classes. Is a charm like a Glamour, where only the appearance of an object changes? Seems not, from the examples above. Explanation?
This is a wild stab, KJM B in Sac: I'd say a jinx is a lasting curse - mild though it can be - that will affect someone when they try to do some particular task or something when someone tries to use it a particular way. A hex affects a person immediately and, depending on the nature of the hex, may take a time to wear off but is not as permanent as a jinx. Neville might well say he is jinxed when it comes to flying - at least in the early books. He has not been jinxed, but he just is. When Hermione says, "Petrificus totallis" in Sorcerer's Stone, I'd say that is a hex.
(Remember, BTW, that "Hex" in German - "Hexe" - means witch.)
I'd say a curse was a hex more than a jinx. To me only a hex is actively applied to a person or thing. I jinx applies to a thing more inately. Harry's Firebolt was tested to see if it was jinxed (I think). When Harry was being tossed around in the air due to Dobby's spell, I'd say his firebolt and the bludger had been hexed.
I had to sort some of this out too, and never got into much detail. Vermis, I think Transfiguration might be easiest to define-- it changes the nature of an object on what we Muggles might call a molecular level, such as the matchsticks to needles, tortoise to teapot, rabbits to rabbit- slippers exercises. It may include some spells termed 'charms' elsewhere; think of the relationship as geology and chemistry, perhaps, rather than math and English.
Charms, as E. Watters points out above, seem to act on objects without changing their real nature-- physical changes rather than chemical, as it were. The twins sell charmed joke quills, but they're still quills and used for writing.
Hexes, jinxes, curses, and counter- hexes et al I find more confusing, but Alan has offered a cogent set of definitions, which I will keep in mind as I re- read the books.
But really, dare I say that an exhaustive compilation of all spells in the HP books might just turn up a few inconsistencies? Naahhh.
Actually, the answer to all of your questions can be found on J.K. Rowling's website! I was confused for a while too, but here's what she said. I just sort of copied and pasted it from her website, so no copyright infringement intended! It starts here...
"Within the Potter world, the boundaries are flexible, and I imagine that wizards may have their own ideas. Hermione-ish, however, I have always had a working theory:
Spell: The generic term for a piece of magic
Charm: Does not fundamentally alter the properties of the subject of the spell, but adds, or changes, properties. Turning a teacup into a rat would be a spell, whereas making a teacup dance would be a charm. The grey area comes with things like 'Stunning Spells' which on balance I think are Charms, but which I call Spells for alliterative effect.
Hex: Has a connotation of dark magic, as do jinxes, but of a minor sort. I see hex as slightly worse. I usually see 'jinx' for spells whose effects are irritating but amusing.
Curses: Reserved for the worst kinds of dark magic."
...and ends here! I hope I could help. Bye people!
Oh, actually, before I go, I wanna show you this quote I found on imdb.com. TOM RIDDLE: By the way, I'm a Parselmouthe. DUMBLEDORE: Oh...well...*beep*