There are two types of digital sequential logic designs possible, including synchronous and asynchronous circuits. The output of sequential circuits factors in both current external inputs as well as past external inputs. This contrasts with combinational logic circuits, which do not feature the memory of sequential logic.
Synchronous sequential logic circuits rely on a clock pulse to signal memory elements to process. This slows down processing to the longest, and therefore slowest, path of the circuit. In contrast, asynchronous sequential logic circuits process inputs as soon as they are received. Although this would result in a faster processing time, it can invert the processing order of inputs depending on which arrives first, potentially creating errors.