Since I was at the panel where they announced this, I'll try to provide some information for context:
-There will be 80 cards total in the set(or there is, tentatively)
-They will have rarities(common, uncommon, rare)
-They are intended to be opened at the table(and I got the implication that each player was intended to have his own stack of the cards) because that way you have no idea what is in your own deck until the random events actually happens.
-The cards are supposed to be random events that happen during a battle. They didn't say what any of the cards in the set actually do, but things like someone tripping over something or a fire starting were possible examples
-The reason they would have rarities is to allow some of the more powerful effects to happen less often. So, you might go through your card deck during a single session, but have only 1 or 2 really strange things happen(the rares)
-The cards are not intended to be collected(they repeated this about 10 times)
-On the other hand, if you happen to collect them all because you buy a booster every week for your session, there will be rules on how to make decks out of them. This is to make sure a player doesn't show up with a deck that contains 10 copies of the most beneficial card for his or her character.