An endomembrane system is a network of membrane like structures inside a cell, consisting of endoplasmic reticulum (rough and smooth), golgi bodies and the vesicles. Each of these organelles have a specified function to perform but are dependent on each other to keep the cell alive.
When enough proteins are made in the endoplasmic reticulum, a sac-like transition vesicle is created which floats through the cytoplasm to the golgi body and is absorbed there.
After the golgi body completes its function on the molecules inside the transition vesicle, another secretary vesicle is created and released in the cytoplasm, further moving to the cell membrane, where the molecules are then released out of the cell.