The endoplasmic reticulum is a series of interconnected sacks and tubes made of membrane. Generically, those tubes help transport materials about the cell.
There are two kinds of endoplasmic reticulum (ER). The rough ER is called that because it is covered with ribosomes. Those ribosomes synthesize proteins that are inserted into the inside of the Rough ER. There, the proteins often go through changes, etc. while being transported to other parts of the cell such as the Golgi Complex. The Smooth ER has no ribosomes on it and is where various fats and phospholipids are made. In some cells, like in your liver, the Smooth ER is involved in the breakdown of toxins, etc. Other specialized versions of the ER are used in cells for storage, such as the modified ER found in muscle cells called the Sarcoplasmic Reticulum that stores Calcium ions for use in muscle contraction.