In Moai SDK, all of the assets (images, tilesets, meshes, and surfaces) are defined in Decks. You can think of a Deck as the geometry of an object. Decks hold the actual image data for a sprite, for example. They're not shown in the screen, until you reference them using a Prop. You can think of Decks as the Prop's drawing method.
There are many types of Decks
MOAIGfxQuad2D, for example, handles one single image.
MOAITileDeck2D
is used to load a tileset (or atlas) and reference its tiles (we will use it later in the book).
Props are basically a collection of information and a reference to a given Deck. They hold information such as location, scale, and rotation; and are the actual objects that you see on the screen.
This separation is pretty useful, since it allows you to reuse the memory allocated to a specific asset. We will use this to handle all of our tiles and instead of loading the image for the back of the tile 20 times, we'll just load it once...