Cards are the most important aspect of Abyssal Horrors in that it allows you to play the game. You assemble your cards into a deck of 40 cards and use it to battle your opponent. There are two distinct types of cards, off-chain and on-chain.
Abyssal Horrors is free to play so everybody will have a set of starter cards off-chain, meaning that they will not be FTs (fungible tokens) or NFTs (non-fungible tokens). The starting cards are built into the game and will let you test the game out before making any purchases, however these cards will have zero value and will not be tradeable. You may also struggle, particularly in multiplayer, as other players will likely own some of the better cards on-chain and have them added to their deck.
On-chain cards are the cards that are FTs and NFTs. It's important to differentiate the FTs and NFTs as there can be duplicates of most cards, which is why they're fungible. Other cards, for example, Hosh'krah from the Founders Set, have only one in existence and are therefore non-fungible.
On-chain cards are stored in your Cardano wallet and can be traded however you wish. The best cards will be on-chain cards, but there will be enough variation in playstyle that there should never be a singular "best deck" as different strategies can be employed in different ways. Please note that Abyssal Horrors can make no promise of monetary value.