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 Recruit keyword.
Summon a minion from your deck.
Recruit is a mechanic that will summon a random minion from your deck that meets the parameter specified on the card (if there are any). Recruit in keyworded form only appears in Kobolds & Catacombs, and cards from other expansions that use this mechanic, such as Y'Shaarj, Rage Unbound or Captain Hooktusk, do not use the keyword. As there are no cards that synergize with the Recruit keyword and the fact that it appears in other sets without a keyword, its presence as a keyword is effectively meaningless.
Keywords that Work Great with Outcast
Rush or Charge minions pulls by Recruit will allow you to make immediate use out of them, and Taunt minions pulled will immediately create a wall which might save you in a pinch.
Playing Against Recruit in Hearthstone
As these decks tend to cheat out large minions, saving removal for them is usually a good idea to take them out before they become a problem. Minions that Recruit other minions are generally very small, meaning that dealing with them is usually not a problem.
Example Hearthstone Recruit Cards
Gather Your Party is a card that can summon absolutely any minion from your deck when you play it. Anything. As a result, you should avoid playing small minions when you play this card. It was also the very first card revealed to use the keyword.
Guild Recruiter is a minion that will Recruit any minion from your deck that costs 4 or less, allowing you to potentially gain a large advantage by cheating out a 4-Cost minion from your deck.
Oaken Summons is the cheapest card that uses the mechanic, recruiting a 4-Cost minion (effectively equalizing the cost-to-card ratio) and giving you 6 Armor. Druuid in particular has strong targets from this expansion including Ironwood Golem and Astral Tiger.
Kathrena Winterwisp is a card central to a "Big Beast" strategy by allowing you to cheat out two huge Beasts from your deck. The card is frequently used to cheat out King Krush to deal 8 damage to the enemy hero and summon an 8/8 minion from your deck.