Card sets are categories into which cards are divided, reflecting how they can be obtained. There are five card sets: Basic, Expert, Reward, Promotion and Missions.
Almost all cards belong to either the basic or expert sets. Basic cards are either automatically included in the player's collection from the start, or are gained through leveling the corresponding class to level 10. Expert cards can be obtained through card packs, or through crafting. Reward cards are obtained through completing certain quests, while promotion cards are available only as part of the corresponding promotion. Missions cards are used by heroes in the game's tutorial and are not available in normal play. Only expert cards can be crafted or disenchanted. Basic and mission cards do not feature the colored gems found on other cards.
There is one exception golden cards. Golden versions of cards from each set are available, but are not necessarily obtained by the usual means appropriate to that set.
Basic is the second largest card set. The vast majority of basic cards are available to heroes of any class. There are 10 basic cards that are only able to be used by their corresponding class. The card known as The Coin is also considered a basic card.
A golden version of each basic card can be obtained by leveling a specific class of hero to a certain level. Raising a hero of the correct class to the required level will 'unlock' the card, granting the player two of the golden version of that card. However, golden cards are still considered soulbound and cannot be disenchanted or crafted.
Expert is the largest card set. Just under half of all expert cards are class-specific, with the rest available to heroes of any class.
There are currently only a few cards belonging to the reward set, namely Old Murk-Eye and Captain's Parrot. Minions created by reward cards are also included in the reward set, although these cards cannot be collected by players. These cards require you to collect all of a certain type of card to get.
There are currently only a few cards belonging to the promotion or 'promo' set, namely the golden versions of Gelbin Mekkatorque, Elite Tauren Chieftain, and the uncollectible cards and minions they create. The regular versions of both these cards are available for crafting. Promotional cards made by Blizzard have a mix of power and flare.
Golden Gelbin Mekkatorque is a reward for any beta testers who help test the in-game store by making a real-money transaction. Golden Elite Tauren Chieftain was available exclusively as a reward for purchasing a ticket to Blizzcon 2013.
Missions is a category used for the computer-controlled heroes (and related special abilities) encountered through the game's introductory tutorial. these battles take place before you start playing vs. other players or stronger AI and are intended to teach you how to play the game. Players cannot obtain the mission cards. There are currently 24 missions cards in total.