Most digital electronic systems have an oscillator that produces a regular periodic two-state signal called the clock that is made available throughout the system. The signal has two voltages between which it alternates—one high and one low per period. The rest of the signals in the system carry just one useful bit of data per clock cycle.
Outputs of many latches are combined in a few levels of logic consisting of boolean functions and these results are the inputs to the latches.
Asynchronous systems, by contrast, lack clocks.