The peritoneum and the peritoneal cavity, parietal and visceral peritoneum

The peritoneum can be considered as a kind of balloon or sac that is inflated inside the abdomen and lines the inside of the abdominal wall. The peritoneum encloses a cavity: the peritoneal cavity.
The peritoneal cavity forms roughly half the (anterior to posterior) depth of the abdomen. The other half of the abdomen lies behind the peritoneum. That is named the retroperitoneal area. To help understand, some structures have been labeled.

Note: It is shown too simplified here. Only the peritoneum that lines the body walls is shown here. Furthermore, the image might give the impression that organs lie freely in the peritoneal cavity which is not true. See the section below.

How are the intestines related to the peritoneum? The classical metaphor to explain it, is to describe the peritoneal sac as a 'balloon' and to imagine the intestines being pushed into this 'balloon' of peritoneum from posteriorly. As a result, a layer of peritoneum comes to lie draped over the intestine.

(In reality it’s just the other way around: in the beginning there’s a solid mass of tissue around the intestine or organ (called 'mesenchyme'). Within this tissue a large cavity forms around the organ, leaving only peritoneal tissue remaining on the inside of the abdominal wall and surrounding the organs. But the balloon metaphor sticks better in the mind.) '.

The layer of peritoneum that lines the body wall is called 'parietal peritoneum' (Lat. paries=wall). The layer that is adhered to an organ is called 'visceral peritoneum' (Lat. viscus=internal organ).  The double layer of peritoneum that connects the body wall and the gut and that contains the blood vessels, nerves and lymphe vessels that supply the gut, is called 'mesentery'.