Synchronous processors have one central clock, which synchronizes and coordinates the progress of operations throughout a processor. Synonym: clocked. All parts and stages work to its unifying beat signal, at the same rate, in lock step, handing off work to the next stage as the clock ticks. Almost all computers made, and in use, are synchronous. Clock speed is a primary performance trait. For personal computers, it is used as an identifier and for marketing, shown in units of megahertz (MHz) or gigahertz (GHz).
Asynchronous processors have no clock. Synonyms: asynchronous logic, clockless, unclocked; self-timed; hand-shaking. All parts and stages work independently, handing off work to the next stage as it is finished. Different stages stop between tasks. All stages can work at the same or different rates. This is more complex to implement, but lowers power use, current peaks, and emitted heat and electromagnetism (raising security). Only a small fraction of computers made, and fewer in use, mainly in embedded systems, are asynchronous.