By selecting a card from the card tab, you can send a card to a friend. This is the ONLY way you can lose a card from your personal collection. And since it's a "free gift" you can't ask Zynga to enforce any trading deals you might have. You can not send a card being used in a squad. Delete the squad first. Don't worry, you can't delete or discard cards.
You can only send one card per 24 hours per friend.
You can not send cards from the Achievement Set to friends. You have to keep them.