Toggle Flip Flop or T Flip-Flop is interesting representation of modified D Flip Flop. Just by having a feedback, D Flip-Flop can be converted to T Flip-Flop. The modification is given below:
The feedback connections make the input signal D equal the value of Q or Q' controlled by an input signal D. At every positive edge when T=0, D=Q and this state will remain same. When T=1 at positive edge clock, D=Q' and will remain unchanged. The overall operation is that: it retains it present state at T=0 and toggles it when T=1.
Basic application of T-flip-flop is a counter circuit.