Rough endoplasmic reticulum[RER] has ribosomes attached to it giving a rough appearance thereby deriving its name. 

smooth endoplasmic reticulum[SER] is devoid of ribosomes. 

RER- involved in protein synthesis as ribosomes are concerned with protein synthesis. 

SER-concerned with lipid synthesis in intestinal cells& 
with steroid formation in adrenals 
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RER(Rough Endoplasmic Reticulum)the outer side of this cell organelle(cytosolic)is studded with ribosomes which is the site of protein synthesis,it is prominent in hepatocyte cell where active protein synthesis occurs,whereas SER(smooth Endoplasmic Reticulum) lacks ribosomes and function in lipid metabolism,carbohydrate metabolism and detoxification,prominent in mammalian liver and gonad cells