Source : Free On-Line Dictionary of Computing

JK flip-flop
     
         An {edge triggered} {SR flip-flop} with extra logic
        such that only one of the R and S inputs is enabled at any
        time.  This prevents a {race condition} which can occur when
        both inputs of an RS flip-flop are active at the same time.
        In a JK flip-flop the R and S inputs are renamed J and K
        (after {Jack Kilby}).  The set input (J) is only enabled when
        the flip-flop is reset and K when it is set.
     
        If both J and K inputs are held active then the outputs will
        change ("togle") on each falling edge of the clock.  JK
        flip-flops can be used to build a {binary counter} with a
        reset input.
     
        {(http://www.play-hookey.com/digital/logic7.html)}.
     
        (1997-07-03)