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.
ETC
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.
~Rio