The Golgi apparatus is also known as the Golgi bodies and the Golgi complex. It consists of a series of flattened membranous sacs called cisternae. (See the figure above. To enlarge the drawing, click the double arrows in the top right-hand corner.)
As you can see in the diagram above, vesicles contains proteins leave the rough endoplasmic reticulum and fuse with the sac of the Golgi apparatus that is closest to the ER.
The Golgi apparatus acts as a processing and packaging center. When proteins leave the rough ER, they exit in little bubbles called vesicles. These transport vesicles fuse with the sac that is closest to the ER. (This end of the stack of sacs is called the cis face, which you can see in the drawing above.
When the vesicle fuses with the sacs, the protein molecules are released into the sacs. While in the sacs, enzymes add additional chemicals, such as carbohydrates, to the proteins. These “tags” specify where the protein (or its modification) will end up when a vesicle exits from the sacs on the far side (trans side) of the Golgi apparatus.