What is Battlecry 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 Battlecry keyword.
Does something when you play it from your hand.
Battlecry is the simplest and most ubiquitous keyword in the entire game. Battlecry effects trigger when a minion with one is played from the hand. They do not trigger if the minion is summoned without being played through card effects such as Voidcaller or Nethrandamus.
Psuedo-Battlecry effects such as Combo, Choose One or Outcast have the same timing and activation rules as Battlecry, but do not count as Battlecries for the purposes of cards like Brann Bronzebeardor Boompistol Bully.
Keywords that Work Great with Battlecry
As Battlecry has timing activation priority over essentially every other keyword in the entire game, as well as no cards that give Battlecries to other cards, there is no one keyword that directly synergizes with Battlecries. That said, as Battlecry minions essentially become vanilla or "french vanilla" minions after their Battlecry goes off, adding any keyword to a Battlecry minion is a positive.
Playing Against Battlecry in Hearthstone
Because Battlecries trigger immediately when played from the hand with no reaction step, they will always activate when played. This means that your best bet to playing against a specific Battlecry minion is to try to proactively play around it.
Boompistol Bully is a card that can delay powerful your opponent's Battlecries for a turn.
Example Hearthstone Battlecry Cards
As Battlecry is an extremely common card (and even with the emphasis, that's an understatment) that does dozens of different things of varying intensities, the examples here will showcase a simple Battlecry, and two different types of Battlecry-related cards.
Novice Engineer is one of the very first cards that the player will see when they start the game up, and appears in several of the Basic class decks. As such, every player will quickly become familiar with this card. It's a simple 1/1 minion for 2 that will draw a card when played. It's playrate is often fringe, but it does occasionally make it into combo decks.
Brann Bronzebeard will make all friendly Battlecry effects trigger twice. As your opponent will typically attempt to get rid of this card as soon as you play it, you should wait to play it until you can take advantage of it. When used this way however, it's often very strong.
Corrupt the Waters is another card that encourages you to use Battlecries, but it works a bit differently. When you fulfill its Quest requirement of playing 6 Battlecry minions, you will gain access to Heart of Vir'naal, which... doubles the power of your Battlecries. Unlike Brann, this Quest specifically asks for numbers rather than raw power, and the effect to double Battlecries requires 2 mana to use each time, but it cannot be removed by conventional means, unlike Brann.