The Endoplasmic Reticulum (ER) is a series of membrane folds. It is the assembly line of the cell. Proteins and lipids are synthesised here and then sent to the Gogli body for packaging and further complicating.
The ER consists of two parts a rough ER (left) and a smooth ER (right). This is due to its appearance under the microscope.
The reason for this rough appearance is Ribosomes. Ribosomes are attached throughout the rough ER.
Another thing to notice is the ERs position in the cell. It is extremely close to the nucleus. As DNA is copied into RNA, it exits the nucleus and travels along the ER to find a ribosome. Without the ER the RNA would need to search for a ribosome in the cytoplasm.