What is Poisonous 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 Poisonous keyword.
Destroy any minion damaged by this.
Poisonous as an ability has existed in the game since launch, but was only made a keyword in Journey to Un'Goro. Previously, cards with the ability read "Destroy any minion damaged by this minion" (as the ability was only applicable to minions at the time). It used to be an extremely rare keyword, but has since appeared much more frequently and its usage has been made for flexible as a result of the keyword.
Cards with Poisonous will destroy any minion that it damages, regardless of how much damage it dealt to it. Poisonous does not affect heroes. Poisonous is mostly associated with Rogue or Hunter, but can be found in any class via Neutral cards. If Poisonous is granted to a card with a damage effect (such as Wild Pyromancer), that damage will also be Poisonous.
Keywords that Work Great with Poisonous
Putting Rush or Charge on a Poisonous minion will allow it to attack immediately, killing something that turn, effectively working as a removal. Divine Shield can also allow it to survive longer, possibly killing more than 1 minion. Taunt will also force minions to attack it, potentially forcing players who control large minions into an awkward situation if they have no other methods of removing it.
Playing Against Poisonous in Hearthstone
Poisonous minions tend to have less stats than vanilla minions of equal cost, and as such, are usually easier to defeat. Because they also require a turn before they attack, they can usually be dealt with before they become a problem. Damage or removal will allow you to remove them without using minions. As Poisonous has no effect on heroes, weapons can also be used to kill them easily. If you have several small minions, you can also "suicide" them into the Poisonous minion, effectively wasting the Poisonous effect.
In the event that you can't remove a Poisonous minion controlled by your opponent though, then do not play any large minions on the board as they will be taken out by the Poisonous minion next turn.
Example Hearthstone Poisonous Cards
Primordial Explorer is a Poisonous minion that will Discover a Dragon will it's played, effectively replacing itself in hand.
Plaguebringer is a Basic card added to Rogue's arsenal after Vanish rotated to the Hall of Fame. It simply gives any minion you control Poisonous. It's also interesting in that it's an effect retroactively added to a past set, as the effect would not be possible to write without the keyword.
Carrion Drake is a minion that conditionally gains Poisonous, gaining it if a minion dies on the turn you play it, but only before you play it as the effect is a Battlecry.
Plague of Madness is an effect that grants both players a Poisonous weapon, meaning any attack your hero deals will kill any minion.