There are three hero factions: the Resolute Faction, the Wisdom Faction, and the Dynamic Faction.
Each faction has a strategic advantage, as you deal increased damage when you use heroes of the faction that counters your opponent, and vice versa, as you gain a bonus when using 2 or more heroes of the same faction in your formation.

Details of the three factions:
The Resolute Faction counters the Dynamic Faction.
The Dynamic Faction counters the Wisdom Faction.
The Wisdom Faction counters the Resolute Faction.

To learn more about them in-game:
-Go to the Hero Hall in your city.
-Tap on Hero Recruitment.
-You’ll find the faction icons at the top right of the screen; press on it for more details