The endoplasmic reticulum (ER) is an organelle found in eukaryotic cells, which plays a crucial role in various cellular processes such as protein synthesis and lipid metabolism. It consists of two membrane-bound sacs - the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER). The RER is characterized by its ribosomes that are attached to the inner surface, which are responsible for protein synthesis. On the other hand, SER does not have any ribosomes on its surface but plays a role in lipid metabolism and detoxification processes.