Synchronous

In digital circuit Synchronous circuits where two circuit elements dependent on each other and change state with every clock signal. Synchronous dominates the digital design.
Clock Signal that oscillates between High(1) and Low(0) used for synchronization.

Asynchronous

In Asynchronous circuits change states whenever the inputs change. that doesn't depend on clock pulse and there is no memory element.

Combinational Circuit

A combinational circuit is a circuit in which make combine the different logic gates in the circuit. The combinational circuit does not use any memory. The previous state of input doesn't have any impact on this state of the present output in the circuit. A combinational circuit will have a number of n variety of inputs and a number of m variety of outputs. For example, encoder, decoder, multiplexer, and demultiplexer


Sequential Circuit

Sequential circuits =Combinational circuit+memory which are dependent on clock cycles and depend on the present as well as past inputs to generate any output and that have memory elements. A clock flip-flops use as synchronous sequential circuits.
There are two types of the sequential circuit 


Asynchronous sequential circuit

If some or all the outputs of a sequential circuit do not change (affect) with respect to the active transition of the clock signal, then that sequential circuit is called an Asynchronous sequential circuit. That means, all the outputs of asynchronous sequential circuits do not change (effect) at the same time. Therefore, most of the outputs of asynchronous ordered circuits aren't synchronous with either only positive edges or only negative edges of the clock signal.

Synchronous Sequential circuit.


If all the outputs of a sequential circuit change (affect) with respect to the active transition of the clock signal, then that sequential circuit is called a Synchronous sequential circuit. That means, all the outputs of synchronous sequential circuits change (affect) at a similar time. Therefore, the outputs of synchronous sequential circuits are synchronous with either only positive edges or only negative edges of the clock signal.