Here's how it works:
By winning games you gain lvls and thus also cards, you can choose wich one you want when you go to the next level. When you start you have 15 cards in your deck, the max is 20 so not until lvl 6 will you have a full deck. You can choose between different self constructed decks, for example: a rush deck, or a natives deck. On your first shipment, you have to choose your deck for the game. Once you chose a deck there will be (offcourse) cards available for shipments, each card can be accessed in different ages and the ages after them, eg: a colonial age card, or fortress age card. If you have enough battle XP (through killing units) you can ship a card to your town-center, for example 600 coin, or a factory. Most cards can only be shipped once and all will be refreshed in the imperial age.
This is everything to the decks and cards system as far as I know. Please do take note that a deck is made to boost you and not hurt your enemy. So your meant to make booming and rushing decks etc. instead of anti-cavalry decks or anti-infantry decks. I hope I anwserd your question as fully as possible, and if someone anwserd it before me it's because I type so slow EDIT: made it more easy to read [This message has been edited by shilly (edited 08-25-2005 @ 04:09 PM).]