Air

Air is literally the absence of a block; It represents unoccupied spaces. Although it is internally treated as every other block, air cannot be kept in the inventory and can only be destroyed or created by the placement or removal of other 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. The sole exception is the Ender Dragon who can move within Obsidian, Bedrock, and End Stone.

Clouds
There are two types of clouds: 2-dimensional and 3-dimensional. Pre-indev releases had 2-dimensional clouds, while post-indev 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. Clouds can be disabled in the video options. Clouds in Minecraft Pocket Edition are similar to Indev.

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.
 * Small boxes of air can spawn commonly in ocean biomes and lakes.