The endoplasmic reticulum (ER) is a network of flattened, membrane-bound sacs that are involved in the production, processing, and transport of proteins that have been synthesized by ribosomes. The endoplasmic reticulum is like the assembly line of the cell, where the products produced by the ribosomes are processed and assembled.
There are two kinds of endoplasmic reticulum: smooth and rough. The rough ER has ribosomes attached to the surface of the sacs. Smooth ER does not have ribosomes attached and has functions in storage, synthesizing lipids, removing toxic substances.