If I remember right, a flip-flop output is always "latched" and on the signal on the rising edge of the clock the output gets latched to the new input and never really becomes transparent. While a latch can becomes transparent when the clock is low, and latches when the clock goes high (level-triggered not edged triggered). You could also call the clock a control signal. Doesn't have to periodic.
Basically,
FF = edge-triggered, never transparent
Latch = level-triggered, can be transparent