The cisternal space, or the interior of the Endoplasmic Reticulum, is continuous with the space between the two membranes of the nuclear envelope. The two different parts of the ER, smooth and rough, have different functions. The rough is so named because of the ribosomes studding its surface; its function, as the ribosomes would indicate, is in the production of proteins. The smooth ER is active in the synthesis of lipids and metabolism of carbohydrates.
Images from Berg, L. 1997. Introductory Botany - Plants, People and their Environment. Saunders College Publishing