1. Sequential circuit is defined as digital circuit whose output is dependent not only on the present input value but also on the past history of its input. The sequential Circuits are designed using the combinational circuits along with a memory devices known as Flip-Flops. The sequential Circuits depend over the input value as well as the stored levels.

2. Flip flop is one bit storage bistable device. Flip flop is also called latch. It stores binary value. It is the basic building block of the digital electronic systems. These are the basically the data storing devices which store the information of two stable states of the system. A flip-flop stores only a single bit of data at a time.

3. The difference between the latch and flip flop is the method of changing their state. The latch do not have clock input but flip flop has a clock input.

4. Counter is a sequential circuit which is  used to count the number of clock pulses of the circuit. It is also sometimes used to display the number of time any event is repeated or happening. It’s a simple counter in the digital logics and computation which calculates the number of times an assigned event is taking place.

5. Register is a group of flip flop or binary cells which holds the binary information. Since a binary cell store a bit of information, n bit register has n flip flops and is capable of storing any information of n bit.