Bricks, technically, aren't an official term - they're more of a nickname in the industry. A box of a dozen decks is stack with three piles of decks side-by-side, laid flat on their backs, four decks high. Put together in a flimsy white box made of heavy paper stock and glued shut, it bares a vague resemblance to a brick in shape, though it's a bit large compared to a typical builder's brick, though smaller than cinder blocks. The weight is similar to that of a builder's brick as well. Occasionally the box has a hole in the lid, permitting identification of the contents without opening it - though such boxes are more common for retail and casino orders than for custom decks. Some custom deck designers will, at extra expense, have a custom box made - often these are third-party manufacture (especially if the deck was made with USPC), better made and have unique art matching the deck's design. Some people are seeking such boxes out, but typically you can only get them when buying a dozen decks. Because of this higher cost threshold and out of a desire to make fans happy, a smaller, "half-brick" box has become popular - the size is two laid-flat decks wide side-by-side and three decks tall. This allows for a custom-made box but a less-expensive package deal for collectors of such things.
...and that is probably more than you ever wanted to know about brick boxes...