A set is a group of cards that adheres to the same theme and are grouped under the same name, but cards from expansion sets are treated very differently to those from Core sets.
Expansion cards (such as those from Trial of the Gods and Genesis sets) have a limited sales run, so the primary difference is scarcity. Once these cards have passed through the Balancing Phase, they are locked to the same stats and abilities for good. With the exception of the Genesis set, expansion cards may be reproduced at a future date but will have noticeable differences – keeping the original set release unique and unchanged.
Core cards, on the other hand, are unlocked by playing the game and available all year round. They are an “open” set in that they can be balanced on the fly, and can be fused to increase their quality.