This is the place where proteins and lipids are prepared to travel to the cell membrane or outside of the cell.
It is the weird folding part close to the cell.
The endoplasmic is often directly connected to the nucleus, so it is very EASY to get mRNA into it.

There are TWO regions of the endoplasmic reticulum:

The rough (RER):
It has ribosomes that are attached to the membrane of the ER
Then the proteins that are created get into the inner space of the ER

The Smooth (SER):
The proteins are transported into the smooth ER.
From the smooth ER, the proteins bud out into a small bubble/membrane in a vesicle, which can then travel to the Golgi Apparatus.
This process can also be called the Transitional ER.
It also detoxifies waste and synthesizes lipids.

The history of the ER:
It is theorized that ER is just an old folded membrane that got stuck inside of the cell and then was used for ribosomes, etc.

The Golgi Body/Apparatus:
The protein vesicles lay inside the Golgi until it is transported into the cell membrane or outside the cell.
There are many enzymes inside the Golgi, which help with the processes and can even tag proteins to where they are supposed to go.

Golgi Protein Functions (Modification):
After getting the protein vesicles, the Golgi body folds and changes the proteins so they are functional.