A eukaryotic cell is sort of like a quaint little self-sustaining village, home to a variety of different organelles providing valuable services that benefit the entire town: a bakery, a mechanic, a grocery store and a mayor. Cells have little stuff-doing structures called organelles that serve specific purposes just like the specialists in a community.
The endoplasmic reticulum, found in eukaryotic cells, is a network of tubes or flat sacs — kind of like a labyrinth of membranes — that serves as the factory of the cell, manufacturing and packaging up proteins and lipids to send around the cell, and even outside of it. About half of the total membrane surface area in an animal cell is found in the endoplasmic reticulum. Which molecules the endoplasmic reticulum makes depends a lot on what kind of cell it is — for instance, the endoplasmic reticulum in muscle cells store a lot of calcium ions because muscle cells need these to make muscles contract, and organs in the digestive system tend to have cells with an endoplasmic reticulum that manufactures a variety of different kinds of cholesterol.
There are two different regions to the endoplasmic reticulum — rough and smooth — and both are found in both plant and animal cells, and although they appear to be separate when you look at them under the microscope, they're really just different compartments of the same organelle.
The rough endoplasmic reticulum is composed of an interconnected network of flattened, membrane-enclosed sacs known as cisternae and it appears bumpy because it's studded with little molecules called ribosomes that assemble proteins out of polypeptide chains and package them up to be used by other organelles or membranes within the cell itself or even exported outside of it. The rough endoplasmic reticulum provides quality control for these proteins and further organizes them for shipment out of the factory and to their final destination.
The smooth endoplasmic reticulum lacks ribosomes, so it appears more tubular and less bumpy under a microscope. Its job, much like that of the rough endoplasmic reticulum, is to manufacture and package molecules, but the smooth endoplasmic reticulum also makes lipids and some steroid hormones, and in some types of cells it metabolizes some sugars that attach to the outside of it. Liver cells contain lots of smooth endoplasmic reticulum because the liver plays a big role in detoxification. So, if you've had a few too many glasses of wine, it's the smooth endoplasmic reticulum in your liver that's helping move things along the next morning.