With the introduction of the Summoner Hero DLC for the PC, another one of the obstacles the player will face on each stage is Minion Units (MU). These units are used to limit the amount of minions the Summoner can summon. Typically the stronger the minion, the more minion units required to summon it.
Minion Units are separate from Defense Units, meaning that summoning minions does not use Defense Units; instead, the Summoner has its own set of Units, shown in the HUD (over the health/mana bar). Every Map the player encounters has a set number of Minion Units, equivalent to the number of Defense Units. All Summoners in a match share the MU between them; each player does not have their own MU count.
The fact that Minon Units are separate from Defense Units makes the Summoner an extremely useful hero, and can make levels a lot easier. It can even become vital for the much harder levels in the game.