There are six Factions in the game you can choose to affiliate with in order to archive your goals and each have their own specialty.
Good Factions
These Factions focus on spells and treasure clicks, they are Fairy, Elf, and Angel.
Evil Factions
These Factions focus on steady and passive building production, they are Goblin, Undead, and Demon.
Vanilla Factions
Good and Evil Factions together are called 'Vanilla Factions' and are a combination from both Factions.
Prestige Factions
Part of the Underworld and Ancient Eternals, the Prestige Factions are purchased in addition to, either a Vanilla Faction of the same alignment, or a Neutral Faction and enable the player to combine the powers from both factions. They are Dwarf, Drow, and Dragon.
Neutral Factions
The Neutral Factions were added in the Ancient Races expansion. Neutral Factions are only available once you have reached a certain amount of Gems and fulfilled specific requirements. They are Titan, Druid, and Faceless.
Mercenary Faction
Mercenaries were added in the Afterlife expansion. They can combine the spells and upgrades from all the other factions to create custom cross-faction mercenary builds.