Air

Air, though technically not a block, is usually treated like such. Air is literally the absence of a block; It represents unoccupied spaces. Air can only be destroyed or created by the placement or removal of blocks, respectively. Air and non-solid blocks are the only spaces the player and other mobs are allowed to move within without mods or glitches.

Clouds
There are two types of clouds: 2-dimensional and 3-dimensional. Pre-indev releases had 2-dimensional clouds, while post-infdev releases have 3-dimensional clouds. The player also has the option to turn the clouds from 3-dimensional to 2-dimensional and vice versa via the "fast/fancy" option in the graphics section in the options menu. As of 1.9 Pre-release 5, clouds can be disabled in the video options.

History
In the Indev versions, air affected fluid blocks that were horizontally adjacent to it. If air was touching the fluid block, there was a 2/3 chance the Fluid block would disappear and make the pool recede and a 1/3 chance the fluid block fill the air, making the pool spread.

Trivia

 * It is possible to survive without air, because other non-solid blocks (that aren't water or lava) won't drown or suffocate the player.

Luft Air/es Air Levegő Air 공기 Lucht Powietrze pt-br:Air/ptbr Воздух 空气