The main difference is that rough ER has ribosomes attached which make proteins. The proteins enter the ER to be transported where they are needed.
Smooth ER does not have ribosomes attached. The main functions are making lipids and steroids,metabolism of carbohydrates, and storage of calcium ions.
Muscle cells need both types of ER. Rough ER to produce the proteins for muscle contraction and smooth ER to store calcium ions for muscle contractions.
