In combinational logic output depends only on present input. Combinational logic is also called as 'memory less logic'. e.g., an adder.
A ----|Adder |---- sum
B ----|
At any point of time output sum is dependent only on the present value input A and input B.
In sequential logic output not only depends on the present input but also on the previous inputs (or in really state of system that has been reached by application of those previous inputs). e.g., a binary counter...
clk ----|counter|---- value
The counter increments by one with application of clk signal (0,1,2,3,4 ...). At any instant of time value not only depends on the input (clk) but also on what was the previous value (state of system reached by the application of previous input). So to know the present value one need to know how many time clk has been applied to the system earlier. Sequential logic are called 'memory logic'.
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