The output is determined by

• the present inputs
• the previous inputs
• the circuits can "remember"
• there is a clock signal controlling the timing
• the circuits are synchronous
• the output changes in time to the clock pulses and not immediately any input changes

Example

Digital clocks display a new time on each clock pulse (once per second). The new time depends on the previous time so the clock has to "remember" what the time is. Here is a system diagram of a digital clock.