All flip-flops can be divided into four basic types: SR, JK, D and T. They differ in the number of inputs and in the response invoked by different value of input signals.
A flip-flop circuit can be constructed from two NAND gates or two NOR gates. These flip-flops are shown in Figure. Each flip-flop has two outputs, Q and Q’, and two inputs, set and reset. This type of flip-flop is referred to as an SR flip-flop.