The most commonly used in-game currency is Gold, which you use to upgrade your Heroes’ Spells. When a Spell is ready to be upgraded, this can be done by pressing the upgrade button for the Gold amount indicated.
Gems are used to speed-up Quests or undertake additional Quests. You can also use them in the in-game Shop to buy Gold, Runes, and Spellbooks.
Runes are used to purchase additional Skins for your Heroes when they are unlocked.