Connecting flip-flops in series in a manner that the output of one flip-flop is fed as input to another, we get a circuit called counter, as already mentioned in frequency division circuit. Counters can be synchronous and asynchronous. If we connect the clocks of all flip-flops together, they will change their state simultaneously, and hence are called synchronous. In asynchronous counters, the outputs of previous flip-flops are used as a clock for the next flip-flops and are therefore slower than synchronous ones.
Counters are extremely important in building a processor, for example. During executing instructions, the processor keeps the address of the next instruction in the instruction address register which can be realized as a simple counter. Every time the instruction is executed, the counter increments its value in order to point to the next instruction address in the memory, so it can be fetched and executed.
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