Minecraft wiki entry for Furniture, associated text: To create the map, dig two (one, if using pressure plates) blocks down, then place the redstone torch on the bottom, (unless using pressure plates) and on top, place the sticky piston. Now you have a map that, if using the torch, is on a table, and, if using the pressure plates, will pop up when you walk up to it! The green parts simulate land, and the rest simulates the ocean.
Once again, to cover a larger area, place more torches and sticky pistons in a grid-like structure. (can't use pressure plates for this) You can also use normal pistons to simulate large tracts of ocean.
You can actually put this on the wall, however, if you do, you shouldn't power the pistons.