The endoplasmic reticulum is an organelle found in eukaryotic cells.
About 50% of the total membrane surface in an animal cell is provided by endoplasmic reticulum (ER). The organelle called ‘endoplasmic reticulum’ occurs in both plants and animals and is a very important manufacturing site for lipids (fats) and many proteins. Many of these products are made for and exported to other organelles.
There are two types of endoplasmic reticulum: rough endoplasmic reticulum (rough ER) and smooth endoplasmic reticulum (smooth ER). Both types are present in plant and animal cells. The two types of ER often appear as if separate, but they are sub-compartments of the same organelle. Cells specialising in the production of proteins will tend to have a larger amount of rough ER whilst cells producing lipids (fats) and steroid hormones will have a greater amount of smooth ER.
(https://bscb.org/learning-resources/softcell-e-learning/endoplasmic-reticulum-rough-and-smooth/)