At first glance, the peritoneum and peritoneal cavity seem pretty simple, but some aspects, like the peritoneal ligaments, can be a bit confusing. In order to understand them, we need to go way back to their embryological formation. 
 Once upon a time, in an amniotic galaxy far far away, the embryonic body cavity is lined with mesoderm. As the fetus is developing, the embryonic body cavity becomes the primordial abdominal cavity and the mesoderm lining it becomes the parietal peritoneum which is a transparent, serous membrane that helps to form a closed sac, called the peritoneal cavity. 

As abdominal organs develop, they protrude into the peritoneum, like pushing your fist into a balloon. Your fist represents the developing abdominal viscera and the balloon represents the parietal peritoneum. As your fist pushes into the balloon it is lined closely by part of the balloon and this represents the visceral peritoneum. So, the visceral peritoneum covers the viscera, while parietal peritoneum lines the internal surface of the abdominopelvic wall; and these two layers are continuous with one another.  
The parietal peritoneum has the same blood and lymphatic supply and the same nerve supply as the region of the wall it lines, meaning it is sensitive to pressure, pain, heat and cold. The visceral peritoneum, on the other hand, has the same blood, lymphatic, and nerve supply as the viscera it covers, meaning it is sensitive to stretch and chemical irritation. 

Depending on their relationship with the peritoneum, abdominal and pelvic organs can be either intraperitoneal, retroperitoneal, or subperitoneal.  
Intraperitoneal organs are almost completely covered with visceral peritoneum, but remember they’re not inside the peritoneal cavity. These organs include the stomach, first part of the duodenum, jejunum, ileum, transverse colon, sigmoid colon, liver and spleen. 
 Now, the retroperitoneal organs, also known as primarily retroperitoneal, develop posterior to the peritoneal cavity, outside of the peritoneum, so they’re only partially covered with peritoneum. The retroperitoneal organs include the kidneys, ureters, suprarenal glands, and rectum. 

There are also secondarily retroperitoneal organs where they begin as intraperitoneal but later on in development become attached to the posterior abdominal wall. The secondarily retroperitoneal organs include the second to fourth parts of the duodenum, pancreas, and the ascending and descending colon. 
 Lastly, the subperitoneal organs, like the urinary bladder, are similar to the retroperitoneal organs, except they are located inferior to the peritoneal cavity, rather than posterior to it.  
The peritoneal cavity is a potential space between the parietal and visceral layers of the peritoneum. Keep in mind that the peritoneal cavity has no abdominal organs, it only contains a thin film of fluid that contains water, electrolytes and other substances derived from the interstitial fluid. The peritoneal fluid helps viscera move without friction, allowing for peristalsis, and it also has white blood cells and antibodies to resist infection.  
Quick quiz. Can you remember which organs are retroperitoneal and which organs are intraperitoneal?  
Now we are going to look at some of the peritoneal structures in adults. Let’s first start by looking at the omentum, which is a fold of peritoneum. There’s actually two of them: the greater omentum and the lesser omentum. 
The greater omentum is a four-layered peritoneal fold that hangs like an apron from the greater curvature of the stomach and the proximal part of the duodenum. After descending, it folds back and it attaches to the anterior surface of the transverse colon and its mesentery. These four layers fuse with one another.  
The lesser omentum is a double-layered peritoneal fold that connects the lesser curvature of the stomach and the proximal part of the duodenum to the liver. Posterior to the lesser omentum, there’s a space known as the lesser sac or omental bursa, and the rest of the peritoneal cavity is known as the greater sac.