Officially, a rick of wood just means a pile of wood. However, convention has it that a rick of wood is normally 4 feet high, 8 feet long and about 16 inches wide. This ends up being about 1/3 of a cord of wood.
Many people use the term "rick of wood" to also refer to a truckload of wood.
Because there are no official dimensions for a rick of wood, it is highly recommended that you buy only by the cord, which has a very specific set of dimensions.