The endoplasmic reticulum (ER) is an extensive membrane network found in the cytoplasm of eukaryotic cells that was first reported in 1945. It contains three types of structures - cisternae, vesicles, and tubules - and exists in two forms: smooth ER and rough ER. Smooth ER is involved in functions like steroid synthesis and carbohydrate metabolism. Rough ER has ribosomes attached and is the site of protein synthesis. The ER plays several key cellular roles like transporting materials, carrying out metabolic processes, increasing surface area for reactions, and forming other organelles.