Campaign (PvE) represents single player progression through the game.
Duel (1v1 PvP) is normal PvP and includes exactly two players (of the same level).
Arena (MvM PvP) is a special kind of persistent PvP for players (in the same league).
Inventory lets you spend the gold you earn in battle on stronger characters.
Drag and drop characters from your inventory next to your other characters on the gamefield. You start with a King, if he dies you lose. Try to kill enemy kings.
Each character has an 'attack:health' pair. When a character is placed next to an enemy, all damage is calculated at once. Earn 10g for killing characters, 50g for killing kings.