C'mon folks, lets supply a useful answer here.
To understand what a DMAP is, you need to understand how the original Legend of Zelda organized its data.
As I understand it, the original Legend of Zelda had only three or four "maps" worth of data. This data tells the game what sprites (here called combos) to draw where, what enemies appear on each screen, etc. One map was the Overworld. The other maps were the Dungeons. If you look carefully, you'll notice that the Dungeons in LoZ "fit" with each other pretty well; one dungeon fits into the negative space of another. They're all on the same map, you see. And, more importantly, all of them are built from the same sprites, or combos.
But remember how the colors of those sprites were different? One level was green, one was white, etc? That was accomplished by making sure the game knew what DMAP, in addition to what MAP, the player was on.
So essentially, it works like this.
MAP = A map is just a way of organizing screens. Think of it as a data structure. You draw screens on a map, and the engine takes care of walking between them. This is just storing the combos, enemies, secrets, scripts, all of the stuff as you create it in the editor.
DMAP = This is a variable that tells the game HOW TO TREAT the data you've plugged into the maps in the form of screens. That is, you don't have to tell ZScript what palette to display for each screen. Instead, you draw a bunch of screens in a given palette (press 0 - 9 to switch between them and see how your screen will look) and then you plug in the palette you want into a DMAP. DMAPS handle things that you want to assign to a whole group of screens on a map, like the music to play, the palette to use, what level the player is on, whether the whistle summons the whirlwind, and many other things.
The DMAP has other important features, too. If you want to set up a warp (like a dungeon entrance) then you need to direct the game what DMAP to send the player too, not what MAP.
One big thing to watch out for is that DMAPS usually contain fewer screens than a whole map. A DMAP designates a certain PART of the MAP. Overworld DMAPS will use the entire MAP, and you don't have to worry about any problems. However, dungeon and cave DMAPS only use a 7x7 square of the data on the MAP you choose. If the player tries to walk out of this 7x7 square, he will be blocked. So when you initially set up your DMAP, pay attention to what part of the MAP you have draw the screens on, and make sure your DMAP is centered on those screens. (You can adjust it.)
The other consequence of DMAPS being smaller than MAPS is that, when you set a warp to a particular screen of a DMAP, you need to refer to it by its coordinates within the DMAP, not on the MAP. So, for instance, warping to screen 3,0 on MAP 2 depends on what DMAP you've set up there. If your DMAP is shifted over by 2 screens, your actual warp coordinates will be 1,0.
That should get you started.