[jamaksin]

Blimps ... a gas bag ... no rigid structural members.

Dirigible ... several gas cells ... beacoup rigid structural members.

US Navy ... Blimps - many; Dirigibles - few (USS MACON, ... USS LOS ANGELES (the last - she was scraped).

[Army Air Corps]

According to most definitions, the words are distinguished as follows:

Dirigible: manoeuverable lighter-than-air craft; steerable airship.
Blimp: non-rigid dirigible.
Zeppelin: Rigid dirigible. Also acceptable to use the term Rigid Airship.

Supporting sources: The National Air and Space Museum, Graf Zeppelin & Hindenburg:The Golden Age of the Great Passenger Airships, Dictionary of Aviation, etc...