It may be a little confusing in the beginning to understand what things last from battle to battle and what things only apply during the current fight. Here are a few of the most important ones to be aware of:
Damage
Health you lose during battle doesn’t come back automatically. There are some cards and Relics that heal you, and of course campfires are always an option, but the damage you take in battle will otherwise follow you to all future fights on the map.
An exception to this is when a Champion dies on the battlefield. After you leave the battle, they will be revived with a portion of their health restored.
Additionally, once you defeat each map’s boss, your entire team will be fully healed!
Cards
Cards added to your hand while out in the world, meaning on the map, in an event, purchased from the merchant, awarded after an encounter upon the death of a Champion, or found as treasure are permanent additions to your deck (unless you choose to pay to remove them while visiting the shop).
Cards you choose at the reward screen are also permanent additions.
Cards gained in battle, however, are temporary. There are some cards that generate cards in your hand, for instance. These cards will stay with you for the current battle, but you will not find your deck full of all the copies you generated once the fight is complete.
Additionally, some enemies may add cards to your deck, discard pile, or hand while you fight. Rejoice in the fact that this incredibly frustrating hinderance only lasts during the time you are in that particular battle, and your deck will be restored to the same state you entered the fight with at the end, plus anything you may choose to take from the rewards screen.
Burn after use
Any card that says “burn after use” will be removed from your deck upon playing it (but not upon placing it in the Card Rack, which doesn’t count as playing). This is temporary, and the card will be in your deck and playable again during the next battle you engage in.