The endomembrane system is composed of the Golgi body, and the rough and smooth endoplasmic reticulum (ER), all found in the cytoplasm of the eukaryotic cells. Together these organelles manufacture sterols and proteins specified by the DNA residing in the nucleus, then then they sort and ship these products to their final destination.
mRNA leaves the nucleus and meets the rough ER, upon which are ribosomes that synthesize polypeptides, some of which enter the cytoplasm, others of which enter the smooth ER and become enzymes. Additionally, some of those polypeptides enter the Golgi body and are released into vesicles that remain in the cell. Other vesicles cross the cell membrane and release the polypeptide outside of the cell. The movement of the vesicles recycles the proteins and lipids between the endomembrane system and the plasma membrane. See a good animation with more detail at the link: