What is Discover in Hearthstone?
Hearthstone has many card effects that are common enough that they get their own keyword - a word or two that immediately tells you how the card works. Let's take a look at the Discover keyword.
Choose one of three cards to add to your hand.
Discover is a mechanic that was introduced in League of Explorers. While it isn't the first new keyword added to the game since launch, it is the first keyword added to the game since launch to appear outside of its debut set. It has since seen a largely positive reception from the community.
When you play a card with Discover, you will get a choice of 3 options fitting the various parameters explained on the card. For example, "Discover a spell" will give you a choice of 3 spells. Discover can normally only give you Neutral cards or cards from your own class, although there are a few exceptions that specifically state otherwise (example: Stolen Steel), mechanically dependent on an opponent's resource (example: Drakonid Operative) or if there are no Neutral cards of cards of your class that fit in the parameters (example: Hydrologist if generated by a non-Secret class). Discover effects can also only give you collectible cards unless the pool only consists of uncollectible cards (example: Sinister Deal).
Despite the wording of the tooltip, not every Discover card will add the chosen card to your hand. Free From Amber and Eternal Servitude will instantly summon the minion you choose. Tortollan Primalist and Tortollan Pilgrim will also cast the chosen spell on the field immediately. Sir Finley Mrrgglton and its Saviors of Uldum conterpart will also given you the chosen Hero Power immediately as a Hero Power cannot be added to your hand. As a rule of thumb, if the Discover card simply states "Discover a(n) ______" with no secondary effect attached, it adds the card to your hand. If the Discover effect does have a secondary effect attached that involves placing the card somewhere (such as summoning or casting the card), it will not be added to your hand.
There's currently only one card in the entire game that specifically synergizes with the Discover mechanic: Dwarven Archaeologist.
Keywords that Work Great with Discover
As Discover only generates a card, there's no one keyword that specifically synergizes with it.
Playing Against Discover in Hearthstone
As Discover largely relies on randomness choosing options from eligible pools of various sizes, there's often no easy way to play around the effect.
To play around it optimally, there's a lot to pay attention to that may affect when/if your opponent will play it. First and foremost, pay attention to the parameters of the pool specific on the card. For example, if your opponent plays a "Discover a spell" effect, you know they got a spell. The amount of mana they have and the current board state will also have an effect. If your opponent has a fair amount of mana left, but didn't play the card they Discovered that turn, it's possible they may have gotten a board clear or a really huge minion, so be sure to play around effectively.
Example Hearthstone Discover Cards
Emerald Explorer is a simple, yet very powerful, representation of Discover. A 6 mana 4/8 with Taunt is already not too shabby, but it also Discovers a Dragon as an added benefit, which tend to be large, powerful minions with strong effects.
Primordial Glyph has been an old favorite card for many /sarcasm. Mage is strong with spells and spell generation, so the effect makes sense. However, the card is slightly absurd. Because of the 2 mana discount, you essentially get the spell for free when you cast it and even get to play it earlier than you normally could if you choose a big spell. If you get another Primordial Glyph from its effect, you essentially get a free chance to reroll its effect.
Eternal Servitude is an example of a Discover card where sometimes the card you get isn't added to your hand. Instead, Eternal Servitude gives you a choice of 3 friendly minions that died this game (or less if not that many have died yet), and you get to summon it to the field instantly without paying any mana. Staple card in Big Priest decks.
Archivist Elysiana utilizes Discover in a way unlike many other Discover cards. Elysiana is another card where the Discover card isn't added to your hand. Instead, it basically works as a choice of 3 cards with the Discover interface. You do this 5 times, and then your deck is replaced with 2 copies of every card you chose.