Bosses are like monster cards in that you play them and fight them like monsters. Whatever you can do to a monster you can do to a boss. One difference however is that when you fight a boss and do it damage, the damage stays with the boss and it does not heal its HP when the turn is over. They are more like your character in that respect, but you cannot heal them, like you can your character. Another way bosses are like characters are the character actions on the cards. The actions can be done only if your character is the correct level and you have the correct type and number of cards under your character. Just like the actions on your characters. You can use a boss to up your level in an emergency case, however it is not advisable due to the fact that boss cards are the only card type that has no character action if used as a level up card. The actions it has are only for use when played as a monster. Lastly you can only have one boss with the same name in play at one time. You can have more than one boss in play at once as long as they are different bosses.
There have been 14 Boss cards and 4 NPC Boss cards released so far.