What are units?
These are what make up your board. You can typically purchase units that range from 1 cost - 5 cost from the shop, which offers a semi-random set of 5 units per shop refresh. The player can control what items a unit can hold, as well as where they are positioned. However, players cannot directly control the units after combat begins.
Units available for purchase in shop
Units typically have different base stats, with melee units and frontline units generally having higher health, armor, and magic resist. Higher cost units of the same archetype generally will also have higher base stats than their lower cost counterparts. The same often holds true of offensive stats for more carry oriented champions.
Each unit also has a unique ability, most of which are dependent on mana -- when a unit reaches max mana, it will use its ability. Units can also have a starting mana value, which determines the amount of mana that a unit starts with at the beginning of the combat phase. Starting mana DOES NOT affect subsequent casts after the first ability cast.
Unit traits / origins
Units will also have between 2-3 traits. These are inherent bonuses that will be activated to buff either the entire board (ex. mystics), or the unit itself. These unit synergies are important to create stronger boards. There are often breakpoints to activate traits which will be noted in the trait description, and higher tiers which grant stronger bonuses.
Traits for a given board, shown on the left
Acquiring 3 of a single unit will allow them to combine to form a 2* unit, and acquiring 9 of a unit will combine into a 3* unit. Each additional star level grants the unit +80% (multiplicative) attack damage and health, and upgrades the unit's unique champion ability. Additional star levels also enhance the effect of certain items like Bramble Vest and Locket of the Iron Solari.
Mana, ability power, attack speed, and armor/magic resist ARE NOT affected by star level.
There is a limited amount of each unit in a shared champion pool between ALL players. This means that it is harder to upgrade a given unit if multiple other units are holding that unit. In addition, units which appear in player shops are also temporarily taken out of the pool until that shop is refreshed.
For each champion tier, there are:
1 cost - 29 units
2 cost - 22 copies
3 cost - 18 copies
4 cost - 12 copies
5 cost - 10 copies
The odds of hitting a certain champion cost varies depending on the player's level.
All star levels of 1 cost units can be sold for full value.
For 2 cost and above units, 2*/3* can be sold for 1 less gold than their combined cost (ie. 2* 3 cost can be sold for 8 gold)