Airships were lighter-than-air craft that had been popular for air travel in the early 20th century until the Hindenburg burst into flames. They never regained popularity again until the 21st century.
(Note: The background section of this uses most of the same words as Terra Futura's page on hybrid airships to save time.)
In 1783, the first hot-air balloon ride occurred. There was one problem though. How would a lighter-than-air aircraft move against the wind? In 1852, Henri Giffard built a powered hydrogen balloon which a steam engine. The steam engine was too weak to handle the wind. Steam engines were also too heavy. Even with lighter internal combustion engines, the airships were still too small. The first practical passenger airship was design by Ferdinand Graf von Zeppelin in 1893. For 44 years afterwards, airships were transporting passengers around the world. Then, in 1937, a zeppelin called the Hindenburg burst into flames. It would be a while afterwards before airships went back into passenger service. Hybrid airships running on helium were an early attempt.
A hybrid airship got most of its lift from the helium it carried. The engines were there for the airship to become airborne. With that, hybrid airships could land like airplanes, and maintenance and loading and unloading of cargo were easier. The piston engines could rotate to change direction. Unfortunately, hybrid airships had problems. They were not aerodynamic. Control was difficult. The weather was capable of messing them up. Hybrid airships never became popular. More advanced full airships would take there place.
Tech Level: 11
Advanced airships like the Aeroscraft and Strato Cruiser had a new aerodynamic shape. This made them faster. In addition to the rudder, there was also outrigger wings which provided stability and vector thrust. They could spin. All of these increased efficiency and speed. Lighter materials like carbon fiber composites, carbon nanotubes, and graphene increased efficiency and speed even further. Some airships had solar panels on the roof for extra power. Some airship designs for construction and hotel use had a vertical configuration for more stability, especially since speed was not important. Some horizontal airships had a dynamic buoyancy system on internal cylindrical bladders that were inflated and deflated for more stability. This was expensive and rare. Over the course of the 21st century airships were entering the mainstream again, but there was a problem. The helium resources on Earth were running out. In the late 21st century, helium was replaced with a vacuum.