What is Secret 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 Secret keyword.
Hidden until a specific action occurs on your opponent's turn.
Secrets are a hidden effect that you play and will activate once your opponent performs the action stated on the Secret, then its effect will trigger. Secrets cannot trigger on your own turn (except Competitive Spirit which triggers when your turn starts, before you have a chance to make any action), and will trigger on the very first opportunity they can.
Secrets are available in 3 classes. Hunter, Mage, and Paladin. Rogue would later get their own set of Secrets in Kobolds and Catacombs, but they haven't gotten any others since. Every Secret in a particular class costs the same amount of mana (1 for Paladin, 2 for Hunter and Rogue, and 3 for Mage), as it would otherwise be possible to guess the Secret based on cost. Secrets will glow a different color dependent on which class it belongs to (green for Hunter, pink for Mage, yellow for Paladin, and dark purple for Rogue).
Keywords that Work Great with Secret
As a spell-only keyword, there's no one specific keyword that has synergy with the effect.
Playing Against Secret in Hearthstone
As Secrets do several different things, there are many precautions to take when playing around them. Against a Mage, you will typically want to cast a low-cost spell to play around Counterspell or Mana Bind. Play around Repentance, Snipe, or Mirror Entity by playing a low-cost minion. Attack before playing any minions to play around Explosive Trap or Flame Ward. Consider each class's Secret pool, and if the Secret was generated from outside their deck, it's possible they might have a Secret up that usually isn't played in many decks. If you're playing Standard, be sure you know which Secrets have rotated out of the mode.
Against a Rogue Secret, attack with a small minion adjacent to a larger minion to play around Sudden Betrayal, deal large damage to the enemy hero first to play around Evasion, and kill off a minion that would get the least value out of Cheat Death.
Various cards to counter Secrets exist such as Flare, Kezan Mystic, Eater of Secrets, and Chief Inspector. Note that Flare will be countered by Counterspell.
Example Hearthstone Secret Cards
Noble Sacrifice is a Classic Paladin Secret that will block an attack. Play around it by attacking with your weakest minion first.
Explosive Trap is another Classic Secret, and has been a staple AoE in Hunter since the game launched because it's very efficient and powerful. Play around it by attacking the enemy hero before summoning any minions.
Counterspell is yet another Classic Secret which can potentially stop a spell from going off. Play around it by casting a small spell first.
Mysterious Challenger I had to mention as it's perhaps one of the most infamous cards in the entire game. Initially written off as a gimmick, it proved to be extremely powerful coming down as a 6/6 that played 5 Secrets that worked nicely together and it thinned your deck by 5 cards. The Secrets can be removed by a card like Flare, but even if you do, they still thinned out one sixth of their deck and they removed many of their low-Cost cards.