What is the key difference between the rough endoplasmic reticulum (RER), and smooth endoplasmic reticulum (SER)?

The RER is associated with translation of proteins, including protein folding. It is known as 'rough' because it is has ribosomes immersed in it, which makes it easy to remember the protein function.

The SER is associated with the production and metabolism of fats/lipids. The SER does not have ribosomes in it, and so is 'smooth'.