The endoplasmic reticulum is an organelle within eukaryotic cells (cells that contain organelles and a nucleus). Organelles are membrane- bound parts of cell that have a particular function that contributes towards the overall survival of the cell. If comparing cells to multicellular organisms, the organelles would be like the organs.
The endoplasmic reticulum is a network of tubular membranes that is attached to the nucleus and dispersed throughout the cytoplasm (gel-like fluid that fills the cell). If ribosomes are attached to the outer layer of an endoplasmic reticulum, then it is called a rough endoplasmic reticulum because of the bumpy appearance that the ribosomes give. If there are no ribosomes, then it is known as a smooth endoplasmic reticulum.
The endoplasmic reticulum works with RNA and ribosomes to create (synthesize) proteins and lipids.