Pleura are the membranes around the lungs that separate them from the chest. There are two pleural membranes that enclose the pleural fluid, which reduces friction betwen the lungs and the surrounding structures while breathing.
Fissures are invaginations that divide the lungs into lobes (the right has 3 lobes and the left lung has 2).
The diaphragm is a muscle that separates the thorax from the abdomen. It helps in the expansion and contraction of the lungs while breathing.