A network of membranous tubules within the cytoplasm of a eukaryotic cell, continuous with the nuclear membrane. It usually has ribosomes attached and is involved in protein and lipid synthesis.

Rough Endoplasmic Reticulum makes proteins. It has ribosomes stuck all around it.

Smooth Endoplasmic Reticulum makes lipids and steroids, and in addition, controls carbohydrate metabolism, regulation of calcium, drug detoxification, steroid metabolism, and attachment of receptors on the plasma membrane.