Classes in The Elder Scrolls Legends are listed here. Classes define the Attributes or colors of a deck and come in several different archetypes. Each Class is comprised of 2 colors. Picking a class determines which cards will be available as you move forward with building your deck. Creating an Archer deck means you won't be able to include any Intelligence cards, but you can fill it up with Agility cards, Strength cards and Neutral cards (which can be included with any combination of attributes).
You can choose to forgo the two-attribute deck structure altogether, and craft a deck using a single card color. Certain cards in the game have special Summon bonuses that activate when the next card in your deck shares its attribute. Building a deck with a single attribute ensures you get that bonus.