The 6th game in the dominion series, Cornucopia is a small expansion providing 13 new kingdom cards and a set of "prizes", cards that can be obtained only through use of the Tournament action card. Cornucopia cards tend to reward players for having a diverse deck, such as Menagerie, which gives +1 Action and +3 cards if you have no duplicate cards in hand, otherwise +1 card.
The Dominion rules are extraordinarily comprehensive. For questions regarding how cards interact, if you can, check the individual card entries in the appropriate rulebook(s) and you'll probably find your answer there. A PDF of the Dominion: Cornucopia rules is available here.