What is it:

The fuselage is the main body of the plane, as all the other parts of the plane are in some way or another attached to this! The fuselage carries mostly the fuel tanks with the engines at the rear end.
However, in many planes, such as the SR-71 Black Bird have engines mounted on the wings.
It may some times even house weapons like bombs and missiles inside, though usually the practise is to have them on the pylons on the surface of the fuselage.