For the Pevensie siblings, only a year has passed, but in Narnia time (like dog years, but with Christian undertones) it's been nearly 1,300 years since the children last visited. Summoned by Prince Caspian, the kids return to the magical, talking-animal-filled world of Narnia to help the young heir claim his rightful place at the throne.
But much has changed since their last adventure. The fanciful places they once knew are now dank ruins. The rest of the world is now Lord of the Rings, with kids replacing digital pseudo-midgets, and with a talking lion. That's about it.