How does rough endoplasmic reticulum differ from smooth endoplasmic reticulum?

The two differ in their function, and one is smooth and the other is rough.
The (SER) is smooth, and the (RER) is rough.
Both the SER and RER are large continuous sheets of membrane that fold back on themselves to form an enclosed space (lumen). Think of this as like a cloth bag that has had its opening stitched closed and then the bag screwed-up to make a small ball (see below).
When viewed under an electron microscope (as shown in the image below) the RER looks rough as the surface is coated with . The black dots, the ribosomes, are in the of the cell.
However, the SER looks smooth, as it does not have ribosomes coating the outside.
The above images are sections. It is as if we have cut through the screwed-up bag mentioned above and are looking at the ends of the folds, but really this is still one bag.
The SER and the RER have different functions. The RER is involved in the production of in the cell, whereas the SER may be involved in the storage of ions, in detoxification, or in the production of .
References
Image source – bio.libretexts.org
Image source – organelles2011