Cards have different rarities: Common, Rare, Epic, and Legendary. These classifications represent how difficult it is to obtain each card.

Card Packs also have the same rarities: Common, Rare, Epic, and Legendary.

Green - Common
Blue - Rare
Purple - Epic
Yellow - Legendary

The rarer the Card Pack, the better the chance of getting the rarer cards. Some Card Packs also guarantee at least one card of their rarity, i.e.:

Rare Card Pack: guarantees at least 1 rare card.
Epic Card Pack: guarantees at least 1 epic card.
Legendary Card Pack: guarantees at least 1 legendary card.