The main setting of The Chronicles of Narnia is the world of Narnia constructed by Lewis and, in The Magician's Nephew, the world containing the city of Charn. The Narnian and Charnian worlds are themselves posited as just two in a multiverse of countless worlds that includes our own universe, the main protagonists' world of origin. Passage between these worlds is possible, though rare, and may be accomplished by various means. Narnia itself is described as populated by a wide variety of creatures, most of which would be recognisable to those familiar with European mythologies and British fairy tales.
Lewis' stories are populated with two distinct types of character: Humans originating from the reader's world of Earth, and Narnian creatures and their descendants created by Aslan. This is typical of works that involve parallel universes. The majority of characters from the reader's world serve as the protagonists of the various books, although some are only mentioned in passing depending on chronology. Lewis does not limit himself to a single source of inspiration; instead, he borrows from many sources,including ancient Greek and German mythology, as well as Celtic literature.
The Chronicles of Narnia describes the world in which Narnia exists as one major landmass faced by "the Great Eastern Ocean". This ocean contains the islands explored in The Voyage of the Dawn Treader. On the main landmass Lewis places the countries of Narnia, Archenland, Calormen, and Telmar, along with a variety of other areas that are not described as countries. The author also provides glimpses of more fantastic locations that exist in and around the main world of Narnia, including an edge and an underworld.
There are several maps of the Narnian universe available, including what many consider the "official" one, a full-colour version published in 1972 by the books' illustrator, Pauline Baynes. This is currently out of print, although smaller copies can be found in the most recent HarperCollins 2006 hardcover edition of The Chronicles of Narnia. Two other maps were produced as a result of the popularity of the 2005 film The Chronicles of Narnia: The Lion, the Witch and the Wardrobe. One, the "Rose Map of Narnia", is based loosely on Baynes' map and has Narnian trivia printed on the reverse. The other, made in a monochromatic, archaic style reminiscent of maps of Tolkien's Middle-earth, is available in print and in an interactive version on the DVD of the movie. The latter map depicts only the country Narnia and not the rest of Lewis' world.
A recurring plot device in The Chronicles is the interaction between the various worlds that make up the Narnian multiverse. A variety of methods are used to initiate these cross-overs which generally serve to introduce characters to the land of Narnia. The Cosmology of Narnia is not as internally consistent as that of Lewis' contemporary Tolkien's Middle-earth, but suffices given the more fairy tale atmosphere of the work. During the course of the series we learn in passing, that the world of Narnia is flat and geocentric and has different stars from those of Earth, and that the passage of time does not correspond directly to the passage of time in our world.
The Chronicles cover the entire history of the world of Narnia, describing the process by which it was created, offering snapshots of life in Narnia as its history unfolds, and how it is ultimately destroyed. As is often the case in a children's series, children themselves, usually from our world, play a prominent role in all of these events. The history of Narnia is generally divided into the following periods: creation and the period shortly afterwards, the rule of the White Witch, the Golden Age, the invasion and rule of the Telmarines, their subsequent defeat by Caspian X, the rule of King Caspian and his descendants, and the destruction of Narnia. Like many stories, the narrative is not necessarily always presented in chronological order.
Source: Wikipedia, "Narnian universe", available under the CC-BY-SA License.