Boss cards are cards unique to boss encounters. They may be made available to the player, or limited to the boss itself, but are not available outside of their respective encounters. Most boss cards belong to a single encounter, while others feature in several.
With most boss encounters involving unique mechanics and boss behaviours, it is no surprise that many boss cards are designed to interact with these, and are often instrumental to defeating the boss in question. Not available in regular play, boss cards are free from the constraints of the balancing required for playable cards, and often feature outlandish powers and effects.
The links below provide comprehensive lists of all bosses, boss Hero Powers and boss cards seen in each set of encounters.