The Collector Booster pack is a new product geared toward players looking for more rare and foil cards. It includes:

1 rare/mythic rare with extended art 1 foil rare/mythic rare 9 foil commons/uncommons 3 special-frame cards (showcase or borderless planeswalkers) 1 ancillary card 1 foil token

The "ancillary card" is one mechanically unique card from this set that isn't available in normal boosters -- the Buy-a-Box card, for instance, or exclusive cards from the Planeswalker Decks.