Well quite simply a blimp (or pressure airship to give it its correct technical name) has no internal structural framework to maintain the shape of the ship nor does it have a keel, it is simply a non-rigid airship filled with the lifting gas, helium, and it is the internal overpressure of the gas inside the "balloon" and the strength of the balloon material that jointly maintain the airship shape.
A blimp is what moored balloons are also called, sometimes for barrage(obstruction) sometimes for para training i.e. for jump training. A para's first real jump with a static line chute & without a wire is usually from a blimp moored at between 800 and1000ft.
(The word dirigible by the way simply means airship and has nothing to do with the word "rigid", it comes from the French word diriger, which means to direct, to steer. A dirigible is any gas filled airship that has both a motor and can be steered, therefore hot air balloons are not dirigibles)
A rigid airship (Zeppelin) does have an internal framework around the individual gas cells and so it is this framework & not the overpressure of the gas inside that mainstains the shape of a rigid airship.
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