System cards are cards that serve a specific purpose doing play, but which are not directly available.
There is currently only one known system card: C'Thun, which appears to serve as a proxy minion for 
C'Thun, for the purpose of recording and calculating buffs to the card. It may also be the card that is shown when the player buffs their C'Thun.
System cards are not collectible, or available in any form during play. Their text, stats and other properties therefore appear to be irrelevant, but this may only be true in some cases.