The 3rd installment in the dominion series. Players can now use duration cards to set up their next turn, as well as powerful new attacks to hinder their opponents. It also introduces cards allowing you to set cards from your deck aside only to return later. Seaside adds 26 new kingdom cards.
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: Seaside rules is available here.