Each world (Narnia, Charn, our own world, and so on) has an associated "realm beyond", where people go when they die. Narnia's version is called Aslan's country, the version associated to our own world is called Heaven, and so on. For each world, this appears very much like a better and more wonderful version of the land of the living. They call Aslan's country "the real Narnia", and similarly they see a "real England":
"I see," she said. "This is still Narnia, and, more real and more beautiful than the Narnia down below, just as it was more real and more beautiful than the Narnia outside the Stable door! I see ... world within world, Narnia within Narnia...."
[...] And far out to sea she could discover the islands, island after island to the end of the world, and, beyond the end, the huge mountain which they had called Aslan's country. But now she saw that it was part of a great chain of mountains which ringed round the whole world. In front of her it seemed to come quite close. Then she looked to her left and saw what she took to be a great bank of brightly-coloured cloud, cut off from them by a gap. But she looked harder and saw that it was not a cloud at all but a real land. [...]
"Why!" exclaimed Peter. "It's England. And that's the house itself—Professor Kirk's old home in the country where all our adventures began!"
"I thought that house had been destroyed," said Edmund.
"So it was," said the Faun. "But you are now looking at the England within England, the real England just as this is the real Narnia. And in that inner England no good thing is destroyed."
-- The Last Battle
These realms can be thought of as above the lands of the living - or within, if you prefer - but either way, they are not accessible in any way other than by dying.
The versions of heaven associated with different worlds are separate from each other, yet more closely interconnected than the worlds below. Travelling between them is easy:
Suddenly they shifted their eyes to another spot, and then Peter and Edmund and Lucy gasped with amazement and shouted out and began waving: for there they saw their own father and mother, waving back at them across the great, deep valley. It was like when you see people waving at you from the deck of a big ship when you are waiting on the quay to meet them.
"How can we get at them?" said Lucy.
"That is easy," said Mr. Tumnus. "That country and this country—all the real countries—are only spurs jutting out from the great mountains of Aslan. We have only to walk along the ridge, upward and inward, till it joins on. And listen! There is King Frank's horn: we must all go up."
-- The Last Battle
By comparison, as we know, travel between the worlds themselves rather than their "higher realms" is
not easy, and
can only be done by Magic
-- (quote appearing numerous times in the series)
One way of getting between our own world, Narnia, Charn, and so on is via the Wood between the Worlds, which acts as an inter-world meeting place - a transport hub, if you will. But of course it's not the only way: throughout the series we see the main characters go back and forth between worlds in several different ways, usually at Aslan's bidding. What makes the Wood between the Worlds special is that people can move between worlds of their own accord (provided they have rings), and choose which worlds to go to, rather than having to wait to be "summoned" by Aslan/Jesus/etc.
This map of CS Lewis's multiverse might make the situation a little clearer:
Transport between the lower realms is possible using the green and yellow rings (or, at least between Narnia and our world, simply by the will of Aslan - wardrobes, paintings, horns, doorways, and so on). The upper realms are all connected via "the great mountains of Aslan". Transport from the lower realms to the upper can only be achieved by dying.
