The D Flip-Flop - A One-Bit Memory Element

        The combination of two flip-flops constitutes a D-type flip-flop.  That's D because the output of the flip-flop is delayed by the time of one clock pulse.  Check it out.  Set a value for the data and pulse the clock ON and OFF.  You'll find a copy of the data appearing at the output on the trailing edge of the clock pulse.  Now, if we consider the combination of two flip-flops (in the simulation above) as a unit, we have a D flip-flop.  It's called a D flip-flop because it delays the signal.  The signal appears at the output of the circuit delayed by the time of one clock pulse.  Here's another simulation to demonstrate that.  The "blue box" (We don't always use the traditional "black box".) is the D flip-flop.

Simulation - D (Delay) Flip-Flop

        In this simulation, the data input appears at the output of the circuit one clock cycle later.  You can cycle the clock "manually".