A zoo deck is a deck focused around a large amount of low and medium cost minions. Zoo decks generally aim to flood the board with minions, overwhelming the opponent and achieving victory through board control. Zoo decks use mostly cheap minions and efficient trading to dominate the battlefield, before turning their attacks toward the enemy hero.
Strategy[]
The most popular zoo deck is Zoolock. This warlock deck uses 
Life Tap to fuel its rapid output of minions, aggressively seizing control of the board in the early game with minions such as Voidwalker and Flame Imp, and using efficient trades from effects such as Abusive Sergeant and Power Overwhelming to maintain their dominance. Zoolock typically trades the outright aggression of an archetypal aggro deck for smart trades and board control, but will quickly turn its full wrath to the enemy hero once lethal draws near, combining substantial board potential with burst cards like Doomguard for a final push of damage.
Built around a strong and consistent presence on the board, zoo decks often include cards such as Sea Giant, Defender of Argus, Dire Wolf Alpha and Knife Juggler to make the most of their board size. Since zoo decks rely on board presence, area of effect removal such as Flamestrike is very effective in countering them. However, in order to counter these effects, zoo decks often include "sticky" minions such as Harvest Golem and 
Possessed Villager, making it harder for the opponent to clear the board.
History[]
The 'zoo' deck is a term borrowed from Hearthstone predecessor Magic: The Gathering. However, in Magic, a zoo deck is a deck focused on cheap creatures with a high power:mana cost ratio, and was named 'zoo' due to the fact that many of the creatures in the deck were apes, lions, etc. that you might find in a zoo. The first Hearthstone zoo decks were created using simulators during the game's alpha, but the term was popularised by well-known streamer Reynad in promoting his Zoolock deck during the game's beta in 2013.