An airship or dirigible is a type of aerostat or "lighter-than-air aircraft" that can be steered and propelled through the air using rudders and propellers or other thrust mechanisms. Unlike aerodynamic aircraft such as fixed-wing aircraft and helicopters, which produce lift by moving a wing through the air, aerostatic aircraft stay aloft by having a large "envelope" filled with a gas which is less dense than the surrounding atmosphere.
See an illustrated history of dirigibles (and other types of airships) here.
Popular Mechanics, February 1930, the future of dirigibles as aviation experts predicted in 1930
A new generation of dirigibles is being considered by governments and private companies as the price of fuel rises and concern for the environment grows.
In modern usage, balloon refers to any buoyant aircraft that generally relies on wind currents for horizontal movement, and usually has a mechanism to control vertical movement.
Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a solid object.
Why do helium balloons float?
Below are some activities to help you understand dirigibles.
Part I Hover Test: Fill a balloon with enough helium to make it hover. Then, without touching it, move it around the room. To achieve neutral buoyancy, add or subtract paper clips (weight) one at a time. When the balloon floats in the same place for about 5 seconds, you have done it. (If you use a mylar balloon, you can use it many times.)
Part II Air pressure: Drive your balloon by taking a square of cardboard and sharply sweeping it alongside the balloon. Which moves the balloon the best—one swift stroke right next to the balloon? Big swoops? Fanning? Quick swipes?
Part III Challenges: Try moving your baloon around an obstacle (a person or a thing). Organize a race. Observe the challenges of maneuvering your balloon. Compare them to the way a commercial dirigible may be outfitted to maneuver cargo or passengers from one place to another.