Logical Circuits for digital systems can be two types – Combinational Circuits and Sequential circuits. Main difference between the two is that in combinational circuits, the output of the circuit at any time is determined by the current state of input variables only whereas in sequential circuits, the output is determined by the present state of input variables along with the state of memory elements which store information about previous states of the circuit.
Any combinational circuit consists of input variables, logic gates and output variables.
Here, the n input variables take their values from some external source. Similarly, m output variables pass their values to some external source.
A sequential circuit is a logical circuit, where the output depends on the present value of the input signal as well as the sequence of past inputs. While combinational circuit is a function of present input only.
A sequential circuit is a combination of combinational circuit and a storage element.