Question

what does endoplasmic reticulum do in the cell?

Answer 1

endoplasmic reticulum is a network of tubules and flattened sacs that serve variety of functions in the cell.
it constitutes the ROUGH and SMOOTH E.R
the rough is a series of flattened sacs which tends to aid in the attachment of ribosomes to the cytoplasmic part of the membrane.

the smooth is a tubule network which aids in the attachment of lipids

Answer 2

Smooth Endoplasmic Reticulum: Throughout the eukaryotic cell, especially those responsible for the production of hormones and other secretory products, is a vast network of membrane-bound vesicles and tubules called the endoplasmic reticulum, or ER for short. The ER is a continuation of the outer nuclear membrane and its varied functions suggest the complexity of the eukaryotic cell.
The smooth endoplasmic reticulum is so named because it appears smooth by electron microscopy. Smooth ER plays different functions depending on the specific cell type including lipid and steroid hormone synthesis, breakdown of lipid-soluble toxins in liver cells, and control of calcium release in muscle cell contraction.

Rough Endoplasmic Reticulum: Rough endoplasmic reticulum appears "pebbled" by electron microscopy due to the presence of numerous ribosomes on its surface. Proteins synthesized on these ribosomes collect in the endoplasmic reticulum for transport throughout the cell.
http://www.cellsalive.com/cells/cell_model.htm
this is directly copied and pasted from cellsalive.com