The D flip-flop tracks the input, making transitions with match those of the input D. The D stands for "data"; this flip-flop stores the value that is on the data line. It can be thought of as a basic memory cell. A D flip-flop can be made from a set/reset flip-flop by tying the set to the reset through an inverter. The result may be clocked.
Clocked D Flip-Flop:
Clocked D Flip-Flop:A D flip-flop constructed from a NAND-latch .
D Flip-Flop from NAND Latch:
The output Q will track the input D so long as the flip-flop remains enabled.
Ouput: Clocked D Flip-Flop:
The D flip-flop tries to follow the input D but cannot make the required transitions unless it is enabled by the clock. Note that if the clock is low when a transition in D occurs, the tracking transiton in Q occurs at the next upward transition of the clock.