This is an article written by Dave/Zeus (http link here) to answer the question:
I'm going to give you MY understanding of the planes as they are in D&D. Feel free to take it, change it, or leave it as you will.
Imagine a piece of laminated windshield glass from a car. Smash it with a hammer. You see how all the cracks spread out in concentric circles and radiating lines from the point of impact, just like a spider web?
At the middle of the web is what's called the Prime Material plane. This is the plane where all the elements come together to create the world as we know it. All the fantasy worlds you know like Krynn (Dragonlance), Toril (Forgotten Realms, Al-Qadim), and Athas (Dark Sun) exist on this plane. They're all on different planets, of course, but they are part of the same reality. You could theoretically take a spaceship form one to the other if you knew where to look.
(Alternatively, you may see them as parallel dimensions in the same space in the planar structure. This way, you couldn't take a spaceship from one to the other. It's a little more Twilight Zone.)
Moving out from the center of the web a little bit you find the Elemental planes of Earth, Air, Fire and Water. These all exist simultaneously but independantly at the same distance from the center. It's the point at which these four planes overlap that you will find the Prime Material plane.
Each one of these planes will have their own characteristics relating to the element they it represents. For example, the plane of Water will be more or less entirely water. There will be gigantic spatterings of seafloor randomly floating about around, full of corals and vegetation. There will probably be enormous bubbles of air, big enough they would actually simulate the ocean's surface, with all it's storms and swells (and pirates). In those bubbles you might even find small islands and coastlines with or without anything attatching them to the "seafloor."
The plane of Fire will be very hot, with rivers of lava and lots of volcanoes. The "solid" ground itself wouldn't really be ground, but more of a tangible form of fire that you can touch. The sky would be a blazing, swirling inferno.
The plane of Earth will be a lot like the Underdark, only moreso - lots of caves, rivers of mud, and no sky, but rather a rock ceiling with more and more caverns extending on to infinity. All kinds of seams of gems and precious ores would replace the forests and meadows of the Prime Material, and solid unworkable basalt would replace the deserts and swamps.
The plane of Air has very little solid material in it. There's no ground, and there's sky everywhere you look. If you fall, there's nothing to land on. Ever. Some places will be very windy. Here you'll find cities built on clouds, and small mountains housing dragon caves floating in mid- . . . well . . . Air.
Heading out farther still from the center of the web, on each of the larger chunks of glass, you come to the Inner, and eventually, the Outer Planes. These are the realms of the Gods, and all those other things vying for the Celestial thrones.
Generally speaking, the Inner planes are where the Good guys live (that's Good with a capital G) - Nirvana, Olympus, Heaven, Asgard and Valhalla are a few examples. Some of them may reflect the traditional representation of Heaven - lots of fluffy white clouds and fat little angels plucking at harps - while others may not.
The Outer planes are even farther out. Here's where all the Evil guys come from - places like Hades, Hell, the Abyss, Gehenna, Purgatory and so forth. Like the Inner planes, some of these fit the typical representation - lots of fire, rivers of blood, toil, suffering and damnation - while others may not. One of them (I can't recall which) was described to me as a pretty close facsimile of the Prime Material, actually, only with twisted, gnarled trees and buildings, and faces of the dead that would hang just a few feet below the surfaces of rivers so you'd barely notice them as you crossed over a bridge. That's hella creepy in my books.
You could even pencil in the Middle planes, if you want, between the Inner and Outer planes These would be places of Neutral alignment, such as Limbo.
Floating in amongst the spaces between all the other infinite planes you find demiplanes. These aren't entire planes in and of themselves. Some are of a finite space, whereas true planes are each an entire Universe. Some have sketchy or ambiguous laws of physics and such - maybe one day there's a mountain range here, and the next day it's simply gone. Or maybe gravity or fire don't always work the same way twice. Stuff like that. Demiplanes are formed either from chunks that "break off" another plane, from planar material floating in the Astral that congeals, or by powerful Gods or magic-users (maybe even player characters!) that get a little ambitious and try to create a new plane.
The Ravenloft campaign, if you've ever played it, is set in a demiplane. Remember how the Mists would define the very Edge of Existance, and how entire nations would get swallowed up and/or replaced by them? And none of the natives seemed to notice the difference? That's an example of demiplane physics.
Now, look back at the windshield glass you shattered before. You see the thin layer of plastic laminate that didn't break, but rather holds all the other shards in place? That's the Astral plane.
The Astral plane is a sort of metaplane (not demiplane) that connects all the others together and holds them in place. When travelling from one plane to another, you usually need to cross the Astral to get there. I visualize this as a clear starry night sky all around you, where the creatures and objects you encounter are actually living constellations.
You've heard of the "Silver Cord?" When travelling in the Astral plane, every creature has a thin strand of silver, again like a spider web, connecting their Astral body to their point of origin. If that strand is ever severed, they become stuck in the Astral forever, much like a ship at sea that has lost its anchor.
Two more and I'm done . . . .
That about covers it for the greater scheme of the planes. You'll notice, though, that all these planes are on the same sheet of glass. What about lateral variations? This is where the Ethereal and Shadow planes come in.
Go back to the Prime Material plane on your glass. Now, floating a couple inches above it, imagine another similar sheet parallel to it. This is the Ethereal plane. It coexists alongside the Prime Material as an alost perfect replica, and things that happen on one can affect the other directly, unlike the other planes I mentioned which exist independantly.
The Ethereal plane is the place where magic takes shape and pcionicists communicate and do battle. Supposing you were actually in the Ethereal plane, you could see the Prime Material all around you in fuzzy, distorted images. If one of those images began casting a spell, you could actually see clearly the magical energies shaping themselves right in front of you, and you could botch that spell simply by sticking your finger in it.
Also, this is where you can "see" emotions, and change people's motives and feelings through "physical" means, rather than the usual routes.
A person on the Prime Material would see a being on the Ethereal (if that being allowed itself to be seen by way of trying to interact with the Prime) as a ghost.
Now, the Shadow plane. Same thing, but this time imagine the extra glass below the first one. On the Ethereal, things would seem a lot less tangible and real than on the Prime. Here, they seem even more real than on the Prime, if you could imagine that. Breezes are thicker, scents seem almost solid, pain hurts more, and the darkness is more disheartening. Water is black, and not very thirst quenching at all. Basically, your awareness and emotions are in overdrive, and it's normal that way. This would be a great place for a gothic-style adventure.
Now that all that's said, remember that each plane will have its of set of rules and nuances unique to it. Get creative.
For example, suppose your players lob a couple stones over a wall at their foes. Naturally, having done this a million times before, they're expecting the stones to fall back down and hit their enemies. But let's say they're on the plane of Smoochies, where trajectories work a little differently. The stones don't fall, and instead carry on to hit a dragon circling high above the next county!
Or maybe fire isn't quite as hot here as it is on the Prime. Or maybe steel melts at room temperature, or trees grow two feet a day, or sunlight actually does heal wounds.
Then, think of how the new laws of nature will effect the supernatural - i.e. what REALLY happens when the wizard cast fireball or the cleric casts cure serious wounds?
And this will be different all over again for every new plane the party travels to!