What is Endoplasmic Reticulum(ER) And Name its types

What is Endoplasmic Reticulum(ER) ? Name its types? 

The cell organelle, Endoplasmic reticulum

(ER) is an extensive, interconnected, membrane- bound network of tubes and sheets. It consists of various channels like cisternae, vesicles and tubules. 

ER carries materials throughout the cell.

There are two type of ERs: 

Rough Endoplasmic Reticulum (RER) - bears ribosomes, involved in protein synthesis. 

Smooth Endoplasmic Reticulum (SER) - Does not bear ribosomes, involved in lipid synthesis.