I'm not going to tell you anything that wasn't already in a trailer somewhere. The totally awesome trailer, by the way, is the extended preview.
Lyra Belacqua (Dakota Blue Richards): Lyra is one of my most favorite characters in fiction, ever. Brave, bold, emotional, loyal, curious, rebellious--she can lie her head off in a tight spot, but she has a distinct sense of honor.
She's very smart, but not book-learned; I think at one point it's said that she "intimidated" the Jordan scholars who adopted her with her wildness, and so her education was kind of patchy.
My favorite line in the book about her, period, is something along the lines of "He was a murderer, so she knew she could trust him." I still can't quite explain what that means, except that... it's so Lyra. She has her own very distinct sense of right and wrong and a very good heart, and I think that's the key to her character--she believes strongly in what's right, and to hell with anyone who crosses that. If that was my favorite line, my favorite bit in the trailer (extended preview again?) is when she spits at one of the soldiers trying to keep the kids from escaping. I'm pretty sure you see her spit on a Jordan scholar as well, just for fun. That's Lyra.
Her daemon (see more below) is Pantalaimon ("Pan"; voice of Freddie Highmore).
The Lyraverse (as it were): Lyra lives at Jordan College in an alternate-universe Oxford.
Her uncle, Lord Asriel (Daniel Craig), has placed her there with the scholars while he goes off exploring. Otherwise, she's an orphan.
Everything in the Lyraverse is just slightly off--they use "anbaric power" instead of electricity, gyropters and zeppelins instead of helicopters and planes, and the movie has added funky-looking carriages instead of cars (did they use carriages in the books? I can't remember).
England is "Anglia," Norway is "Norroway," and so on (warning: following links to any other article on Wikipedia may result in spoilers). Most strikingly, all people have daemons.
Daemons: Daemon (or in some translations, daimon), not demon. A daemon is a constant animal companion, but it's more than a talking pet; it's you, but in animal form. It's basically the physical embodiment of your soul. A person's demon is almost always of the opposite sex, though, like an animus/anima (awanderingbard points out that Pullman does mention a male cook with a male daemon).
When you're a child, it can change shapes at will (because your own personality isn't set yet); after the onset of puberty, it settles into a permanent shape. The daemon can't go very far from its person--I forget exactly, but it's something like 10-15 feet. Separation is physically painful, and you never, ever touch someone else's daemon. (Let's put it this way: it's such an intimate gesture that it either has threatening [as shown below] or sexual connotations in the books.)
Exception: due to the nature of witches (more below), their daemons can travel unlimited distances apart from their person. Otherwise, seeing a person without a daemon present is like seeing someone horribly mutilated.
The Alethiometer ("Golden Compass"): It's described as a "golden compass," but it actually "tells the truth"--not "shows a glimpse of the future," TRAILER. Obviously, if you ask about the future, it will tell you the truth, but it's not just the future. You ask it a question mentally and it will point to various symbols as an answer, but each symbol has hundreds if not thousands of layered meanings, and so the answers can only be deciphered by someone who has studied the symbology, usually for years. The reason Lyra is "meant to have it" is because she turns out to have a natural gift for reading it. I think something along the lines of "the grace of a child" is mentioned, but... Lyra has a unique kind of "grace," let's put it that way. She uses her powers of brattitude for good, I guess.
The Magisterium: Sort of a governing religious body (I forget exactly how far into the government it reaches) that's... well, in the movie, Christopher Lee is on it, that's pretty much all you need to know. I'd get into their actual beliefs, but that'd be giving things away.
Gyptians: Lyraverse gypsies (Roma) are water-traveling traders, rather than terrestrial wanderers; in "Anglia," they travel the rivers and canals. The Gyptian families are far-flung but close-knit, and hold councils headed by a king--at this point, John Faa. Farder Coram is another Gyptian in the books, a wise counselor who once... well, I'll let the movie get to that. The Oxford kids have "wars" with the Gyptian kids--mudfights, throwing bricks at each other, that kind of thing. I think Lyra makes off with a Gyptian boat in her childhood? So Lyra's somewhat familiar with them, and they become her first allies.
Witches: They live for hundreds of years, are all female (their male children are simply human) and own nothing, except for their cloud-pine branches, which they fly like brooms. (In the movie, they seem to fly on their own, but carry rough tree-branch bows. Bows are their weapon of choice in the books as well.) Flying is most important to them; I think the line in the book is something like, "To fly is to be perfectly ourselves." Usually they wear "scraps of black silk"; cold does not harm them. Serafina Pekkala (Eva Green) is the queen of the Lake Enara (Finland) witches and becomes close to Lyra over the course of the trilogy; her daemon is a goose named Kaisa. Generally the witches are fairly stoic and unemotional--somewhat like Tolkien's elves, and probably for the same reason: extremely long life--but Eva Green's Serafina comes off as a lot warmer in the trailers. Which is fine by me.
Panserbjørne (literally, "armored bears," a name they also go by): They have no daemons, but rather find their identity in their armor, which they smith from "sky-iron," or meteorites. Armored bears, by the way, have opposable thumbs, and are even better than humans at smithing metal. They are described in the book as having a very non-human quality, as opposed to simply being bears that talk like humans (the daemons, for example, act and speak in very human ways), which was my chief problem with having Iorek Byrnison voiced by Ian McKellen, because McKellen just sounds too nuanced and human. The voice in the previous trailers, Nonso Anozie, had this wonderful, gruff, almost metallic quality. Anyway, Iorek develops a relationship with Lyra and, later, challenges another bear, Iofur Raknison (renamed to Ragnar Sturlusson in the movie, which I can understand, because if you can only hear the names, "Iorek" and "Iofur" would probably confuse people) for the throne of Svalbard and the rule of the bears. I mention this because that's the climactic fight you see in the trailers ("IS THAT AAAAAAALL?"). Iofur/Ragnar is voiced by Ian "Old English" McShane, by the way).
Other characters, in general order of appearance, and the (vague) plot that goes with them: Lyra's best friend is a boy named Roger; he's kidnapped by the "Gobblers," which you see in the trailers, and Lyra swears to find him again.
Lord Asriel (Daniel Craig), as mentioned, is her high-handed explorer uncle; his daemon is a snow leopard (voice of Kristin Scott Thomas). Lyra also discovers that her uncle's been captured on his explorations up north, and she'd like to help free him as well.
Mrs. (Marisa) Coulter (Nicole Kidman) is a very charming, elegant, sophisticated--my first instinct was to say "socialite," but she does work in the government and/or church, as well as being involved with Jordan College, and the most recent trailer informs us that "she's in charge of the Magisterium!" Her daemon is
Initially she goes to Jordan College to ask Lyra to be her "assistant" as she prepares to travel north herself. (Yes, apparently up north is where it's all at.)
After Lyra discovers that Mrs. Coulter is behind Roger's kidnapping (okay, the trailer doesn't specifically say that, but I'm trying to connect the dots they do give you), she runs away and is rescued by the Gyptians, who have also had a number of children stolen. Together they go looking for the children up north, where they've heard that terrible things are being done to the kids.
There, on their way up north, Lyra meets the witch Serafina Pekkala (again, Eva Green) and aeronaut Lee Scoresby (Sam Elliott), who is going to be everyone's cowboy grandpa after this movie. His daemon is Hester, a jackrabbit (voice of Kathy Bates). He agrees to help the Gyptians and bring his hot-air balloon, suggesting to Lyra that she might want to engage the services of an armored bear, and he happens to know one who might appreciate some help in getting his armor back. Enter Iorek (now the voice of Ian McKellen), who now "has a contract with the child."
We're about halfway through the book now and I'm not telling you any more, except that obviously Lyra does find the missing children and there are clearly some shenanigans going on several sides: Mrs. Coulter is chasing Lyra to get her back, Serafina and Lee and Iorek are trying to help her, and she may or may not find Roger and/or her uncle as she intended.
Suffice it to say, the trailers are spending most of their time trying to outline what I'm telling you here, and they're truly not giving away that much plot; I think New Line's more concerned that if they don't do a lot of explaining up front, they won't be able to get people in the theater. And none of this gets into the religious controversy of the books, probably because those elements don't really become apparent until the second one. I'm personally open to whatever changes they try to make--at least to see what they try and if they pull it off--because this adaptation, from the word go, is obviously so much more appreciative of the source material than, let's say, The Seeker. I can forgive a lot if you've got good intentions.