There are several types of currencies in the game and they are each used in a different way.
Gold
This is the basic currency of the game and it allows you to buy the basic card packs to build up your deck. Gold can be earned for winning matches and completing Challenges.
Gems
This is the premium currency of the game and they can either be purchased using real money or earned by winning matches, completing Challenges, and participating in events. Gems can be spent on buying premium card packs, unlocking new campaigns and trading them for gold.
Shards
You will be rewarded shards when you win a match and their only purpose in the game when you have collected enough of them is to open the special chest in the home menu. This special chest will contain a decent amount of loot.
Inks
Inks are needed to craft new cards and are obtained by disenchanting cards.