What is a bridge card and how is it played?

According to Contract Bridge Law 45, a card is played if

• it is by a defender such that his partner could see its face
• it is by declarer face up, touching or nearly touching the table, or in such a position as to indicate it has been played
• it is in dummy deliberately touched by declarer (except to arrange the cards or to reach above or below the card touched)
• any player names the card he is playing (i.e. if you say "the six", the six is considered played)