At the core of The Adventurer's Guild is combat. Battles in The Adventurer's Guild are turn-based.
The first combatant to lower their opponent's HP to 0 wins the encounter.
Taking your turn
At the start of your turn, if you're below maximum energy, you will recover some energy.
On your turn you can do one of the following:
Attack with your profession's basic attack (costs 0 energy)
Use a skill (generally costs energy)
Use a consumable, such as a potion
After you do your action, if you have a companion or pet, they will take an action depending on your charisma.
Then your opponent will take their action, and the cycle will continue.
After the encounter
If you win the encounter, you will earn some experience and guilders. Depending on the opponent you defeated, you might also find some loot!
If you were defeated, you will fail the quest you are currently on, and will need to start the quest again. You will also need to wait a period of time to "revive", emulating the need to return to Guildtown for healing. The revival time is 30 seconds/level. Some potions will allow you to instantly revive, but they will be difficult to obtain.