Instead of choosing a class and getting a bunch of Level 1 abilities, you are given cards that represent different abilities, features, equipment, etc. and equip them into your character's 'deck'.
At Level 1 you have a deck size of 6 cards. You're limited to 1 Weapon card, and 1 Armor card, the other 4 can be for your choice of skills, traits, and other bonuses.
Leveling up doesn't increase your stats, but instead increases the number of cards you can put into your deck. You always have manual control over every aspect of your character, and can change them at any time. Changing from a Knight to a Wizard is simply a matter of swapping out your cards.
Cards can be bought in themed packs so you always know what kinds of cards you'll be getting. A Wizard Card Pack will always contain cards that have to do with spellcasting.
You can also get cards from basic enemies, but they're usually weaker versions of what you get from packs.
The best will come from dungeon bosses and quests.
Rarity also impacts the effectiveness of a card. So, a Common HP Increase card will give you 50 additional health, while the Rare version of the same card gives you 175.
Having an entire armor set on one card does limit your ability to customize your appearance, especially if there is one definitively strongest set that all endgame players will be wearing, which is why you can put Armor cards into Sleeves that give it a different appearance. Turn an armor card you like into a sleeve, and put your new, superior armor card into it, and it will look like your old armor when equipped. Special sleeves can be used to change the color of your armor as well. Same logic applies for weapon cards.