The endoplasmic reticulum (ER) is an network of sacs that manufactures, processes, and transports chemical compounds for use inside and outside of the cell. The ER is a continuous membrane with branching tubules and flattened sacs that extend throughout the cytoplasm. It is connected to the double-layered nuclear envelope, providing a connection between the nucleus and the cytoplasm.
There are two kinds of ER, rough and smooth. Rough ER is covered with ribosomes, giving it a bumpy appearance when viewed through the microscope. This type of ER is involved mainly with the production of proteins that will be exported, or secreted, from the cell. The ribosomes assemble amino acids into units of proteins, which are transported into the rough ER for further processing. Once inside, the proteins are folded into the correct three-dimensional conformation, as a flattened cardboard box might be opened up and folded into its proper shape in order to become a useful box. Chemicals, such as carbohydrates or sugars, are added, then the ER either transports the completed proteins to areas of the cell where they are needed, or they are sent to the Golgi apparatus for export.
Smooth ER has a smoother appearance than rough ER when viewed through the microscope because it does not have ribosomes attached to it. This portion of the ER is involved with the production of lipids (fats), carbohydrate metabolism, and detoxification of drugs and poisons. Smooth ER is also involved with metabolizing calcium to mediate some cell activities. In muscle cells, smooth ER releases calcium to trigger muscle contractions. Cells specializing in lipid and carbohydrate metabolism (brain, muscle) or detoxification (liver) usually have more of this type of ER.
Information provided by: http://micro.magnet.fsu.edu