Endoplasmic Reticulum is the site of synthesis of proteins and other molecules destined for other components of the Endomembrane System and for secretion to the extracellular space. The ER membrane is continuous with the Nuclear Membrane and has two components:
Rough ER contains many bound ribosomes that synthesize proteins.
Smooth ER has no bound ribosomes and is the site where vesicles “bud” off to transfer protein and membrane to other organelles.