Endoplasmic reticulum (ER) is a membrane-bound network extending from the nuclear membrane to the plasma membrane. When ribosomes are found attached to the surface of the ER, it is called a rough endoplasmic reticulum (RER) and if there are no ribosomes attached to it, it is called the smooth endoplasmic reticulum (SER).
SER is involved in the synthesis of lipids and steroid hormones.
Ribosomes on RER are involved in the synthesis of proteins.