In Gwent, cards are divided by their Rarity levels, among other factors. To know which group a card belongs to, you have to look at the little square in its bottom right corner.
Common cards can be recognized by a white square in their bottom right corner. They are the most frequently found cards in Kegs so you'll have a plenty of them very soon. Because of their multiplicity, you will get a lot of copies, which can be annoying, but at least you can mill them to get Scraps and craft more useful cards.
Rare cards can be recognized by the blue square. They are more valuable than Common cards and their amount in kegs is limited. Every keg guarantees you at least one Rare card so it's harder to obtain them.
Epic cards have a purple square and are very rare. They are much more valuable and powerful than Common and Rare cards. You can only have one Epic card in a deck.
Legendary cards are marked with a golden square and a golden border. They are the most valuable both in terms of additional skills and their collectible value. Legendary cards are incredibly rare - if you're unlucky, you might have to open a lot of kegs before you get one.