What is the difference between Smooth ER and Rough ER?

Rough ER is involved in protein storage and synthesis, and tends have higher density around the nucleus. It has ribosomes bound to the cytosolic side of the membrane which are responsible for translation. Proteins are also folded in the lumen of rough ER.

Smooth ER is mainly involved in lipid metabolism and steroid production, as well as lipid storage. In muscle cells, smooth ER also plays the critical role of storing and releasing calcium ions. Unlike rough ER, smooth ER does not have ribosomes bound to its surface.