What is Combo 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 Combo keyword.
A bonus if you already played a card this turn.
Combo is a mechanic found exclusively on Rogue cards, granting them a bonus effect if they've previously played another card that turn. As Rogues have the lowest average mana Cost of class cards out of every class in the game, their tendency to play several cheap cards, and their strong drawing abilites, Combo effects tend to be easy to activate, especially in the late game. Rogue also gets the largest benefit from The Coin more than any other class.
Keywords that Work Great with Combo
As it is a pseudo-Battlecry effect, the best mechanics that work with Combo aren't keywords, but rather mechanics. Rogue's ability to "bounce" cards back to their hand is often a strong way to pair combos together.
Combo effects that scale based on card count (such as Edwin VanCleef) can work really well with cheap Echo cards.
Playing Against Combo in Hearthstone
Combo effects will happen immediately, so there isn't much of a way to play around the mechanic itself.
Edwin VanCleef is a classic (both literally and figuratively) playmaker for Rogues, and an early one can seal the game as early as turn 3. Against a Rogue, it may be wise to keep a cheap source of removal in your opening hand so you have a way to deal with Edwin should he hit the board early.
Example Hearthstone Combo Cards
Defias Ringleader is a classic (I swear I'm not doing this joke on purpose) Rogue Combo play, providing a 2/2 and a 2/1 on the board for 2 mana. Playing it on turn 1 with The Coin can have some pretty strong board presence so early in the game. Although it doesn't see play anymore, it has one of the best voicelines in the game, "This is our town scrub.", "yeah beat it."
Edwin VanCleef is a staple Legendary that easily slots into many Rogue decks. If played early in the game, one can play an 8/8 or even a 10/10 on turn 3. This is something that not every deck is capable of dealing with that early, and even those that can are not necessarily guaranteed to have that removal in hand.
Eviscerate has been a staple Rogue card since launch. Although dealing 2 damage for 2 is quite weak, being able to deal 4 damage for 2 is anything but. The Combo is fairly easy to activate, and the card is strong as removal or as burst.
Cursed Castaway doesn't have a Combo effect, but instead interacts with the mechanic outside of using it (and is the very first card in the entire game to do so). When it does, it will tutor a Combo card out of your deck.