Chests are standard Vanilla items capable of holding 27 stacks of items, and twice that amount in a double chest.
When you break them they drop all their contents.
They are crafted by making a ring of planks, same as the recipe for the furnace (only furnaces use cobblestone).
You turn a regular chest into a double chest by placing them directly next to each other. If you break it in half it will drop its items no matter where you put them in the double chest's inventory.
A chest cannot be placed directly next to a double chest.