A map is made up out of a number of gridslots. Since maps are always rectangular, the number of gridslots is equal to the number of columns times the number of rows, even if not all of those gridslots are used.
A gridslot is made up out of a number of layers (from top to bottom) :
Entity : a multipurpose object that can contain various triggers or behaviours - it could be a wall, a door, a trap, or even an NPC.
Item : an object that players or NPCs can pickup or use.
Decal : decals allow to add details to the background layer, this layer can be targeted by a trigger or behaviour.
Background : the floor or the space background, which is either a simple image, an image with a detail layer or an image controlled by a randomizer style.
Each of the above layer objects are all tied to a single gridslot. There are also objects that affect multiple layers at once :
Randomizer : a randomizer is tied to a background layer and can be configured to randomly add detail to otherwise plain looking background tiles.