Rarity is a way to describe how hard to find and good a card is. Cards of higher rarity are typically more powerful or more useful, but are much harder to find and more expensive to craft. However more common cards can be disenchanted to get Arcande Dust, which can be used to craft rarer cards.
Free is the lowest level of card rarity, below common, rare, epic and legendary. All free cards belong to the basic set, and therefore do not have any colored gems at the bottom-center of the card's art.
Two of each free card are automatically included in the player's collection.
Common is the second-lowest level of rarity, above free. Common cards that are not from the basic set can be seen with a white gem located at the bottom-center of the card's art.
Rare is the level of rarity above free and common. Rare cards can be seen having a blue gem located at the bottom-center of the card's art.
All the rare cards can be obtained by using the Crafting ability.
Epic is the second-highest level of rarity below legendary. Epic cards have a purple gem located at the bottom-center of the card's art.
Epic cards can be obtained either through Crafting or by opening up card packs bought from the Hearthstone store. However some Epic cards come as rewards for completing tasks.
Legendary is the highest level of rarity. Legendary cards have an orange gem located at the bottom-center of the card's art. Legendary cards also have the special "Elite" Dragon around the outside of the card from World of Warcraft.
You may only have one of each legendary in your deck. In addition to being very powerful cards, the characters and items featured in. Playing a legendary card prompts a short piece of unique music.
All the Legendary cards can be found inside of card packs or created through crafting.