Chapter 10 discusses combinational devices, which are commonly used in computer design. As useful as these devices are, however, it is impossible to build even the smallest computers as an interconnection of combinational circuits. In all the combinational devices mentioned, the output depends only on the input. When the input changes, it is only a matter of a few gate delays before that change is reflected in the output.
The state of the circuit is the characteristic that distinguishes a sequential circuit from a combinational circuit. A sequential circuit can remember what state it is in. It has memory, in other words. The output of a sequential circuit depends not only on the input, but also on its state.