Air is not exactly a block, but it is a representation of the absence of a block. There is no way of obtaining it, destroying it, or creating it. Air fills the void when a block is destroyed by a pickaxe or fire. It is completely transparent, therefore not considered a block, as it cannot be seen, placed, destroyed, set on fire, etc. It can only be "removed" when a block is placed down. Both normal and flat worlds contain air.