What is a kong, and when can you declare one?

A kong is a set of four identical tiles. You can declare a kong in three ways: a melded kong (claiming another player's discard to complete a set of four), an added kong (adding a self-drawn tile to an existing melded pung), or a concealed kong (drawing all four identical tiles yourself, declared face-down). After declaring any kong, you must draw an extra tile from the back of the wall to maintain a 13-tile hand before discarding.