Oh, Cores are the thing that decks are based around. They are usually 1/2 Pokemon that work together well in a deck that really keep it flowing.
For example, In Reshiboar, the cores would be Reshiram and (ability) Emboar.
In Klingklang/Coballion it's, well, Klingklang and Coballion.
Darkrai or Mewtwo decks consist of Darkrai EX and Mewtwo EX.
Eels are Eelektross.
So yeah, pretty close to the video game. You build a deck around a strategy, and the Pokemon that carry out that strategy, and then construct the deck to better that strategy and make it flow correctly.