The force that causes objects to float. According to the principle of Archimedes, when a solid is placed in a fluid (a liquid or a gas), it is subject to an upward force equal in magnitude to the weight of the fluid it has displaced.
How party balloons and blimps achieve lift is by replacing an area of air with something lighter than air, which would be Helium or Hydrogen.
Back in history when the Zeppelin company was building it's large airships the only two countries which could produce Helium was the United States and Great Britain. As Germany was using Zeppelins to Bomb England and the United States was Britain's ally neither would sell or provide Helium so the Germans were forced to use what they had available, which was hydrogen. The Primary difference between Helium and Hydrogen is that Hydrogen is highly combustible where as Helium is not. Hydrogen also is lighter than Helium therefore able to provide slightly more lift per square foot or liter of gas used to displace the air.
Another side note is that knowing that the Germans where using Hydrogen in the Zeppelins and needing a way to ignite the gas inside the airships with a bullet - the tracer round was invented. A bullet with a tail that burns as it flies through the air. Because of the fact that you could see the round fly through the air the use of tracers has been used since then generally as every third round in belts of ammo for chain feed machine guns. Watching these guns fire is much more exciting with the tracers flying down range, then when you consider there are two rounds in between what your seeing that you can't see. WOW.
I will end this post with questions.
What's lighter than Hydrogen?
What's lighter than nothing?
What's the maximum percentage of a vacuum we can achieve today using vacuum pumps to pump the air out of something?