What are endoplasmic reticulum?

They are a part of the group of membranous organelles named endomembrane system, that comprises ER, Golgi apparatus, endosomes, lysosomes, nuclear membrane (envelope) and vacuoles. This system is involved in coordinated functions, such as synthesis and maturation of carbohydrates, lipids and proteins, sorting , targeting and packing of the synthesized material, and a role in bulk transportation (endo and exocytosis) and intracellular digestion and signaling, Mitochondria and chloroplasts are not a part of this system since their membranes are independent to the other cytoplasmic membranes.