Units are what you control in the game. You build units to make an army to defeat the opponent's statue. Every Empire has 9 units, making 27 in all.
Elementals have only 9 units (excluding chomplers and scorplings) even though their unit creation screen has 11 icons, because Scorch and Blind Gate are not units. However, they are still featured on this page because they are still formed from Elemental combinations. Scorpling is not a unit that can be bought with gold or mana, but it is spawned by Treatures, so it is still featured here.