First of all the most important: DO NOT WATCH THE MOVIE! Or if you have: Please try reading the books, they are so much better and actually make sense. I don't what happened at the movie. It was awful! I was so angry afterwards ... even Nicole Kidman didn't rescue it, and she normally can!
Right, let get to the nicer things in life: The books! Awesome!
The story takes place in a parallel world to ours. It's a really cool world. The people living there carry their souls in form of animals with them. As long as they are children and haven't yet reached puberty the animal will change as much as the character of children do. Really interesting. So the story is about Lyra who has to save the world. She does that with help of her soul (demon), the coolest polar bear on the planet and Will, who comes from our world.
It's a hard story to explain ... but once you're into the story everything makes sense. It is so well written than do really feel with the main characters. I personally would not suit the book for 'from 12 years onwards' but like 16 years onwards, because it does get complicated every now then.
It is kind of the simple story of good against evil, but it's told in such a beautiful and astonishing way! Just the creation of the parallel world, all those characters and epic places! Wonderful!!
And a short warning in the end: There is a scene in the 3rd part where I cried like a baby. I actually had to take a break to calm down again. And I am not the only person who did that!