This is a great question. First, it is important to state that there are two types of Endoplasmic Reticulum (ER). There is the rough ER and the smooth ER and they are called as such by the way they look. The rough ER has a bumpy quality (the rough ER is studded with ribosomes), whereas the smooth ER looks more like tubes.
Second, in terms of function. The rough ER can be seen as a packaging center, where proteins are synthesized. Some of this protein used right away and some of it is sent out to be used by other cell through the Golgi apparatus. The smooth ER can be seen as a storing station. It stores calcium ions, lipids, muscle cells, and the like.
Another important function of the smooth ER is the ability to detoxify drugs.
Rough ER - Protein synthesis.
Smooth ER - Lipid synthesis.