The "Plan-Do-Check-Act" cycle was first developed in the 1920's by Walter Shewhart, and was popularised later by W. Edwards Deming. For that reason it is often referred to as "The Deming Cycle". Extensive literature exists about the PDCA cycle in numerous languages, and users of the ISO 9000:2000 family of standards are encouraged to consult this for a deeper understanding of the concept.
The PDCA concept is something that is present in all areas of our professional and personal lives, and is used continually, either formally of informally, consciously or sub-consciously in everything we do. Every activity, no matter how simple or how complex, falls into this never-ending pattern:

Within the context of a quality management system, the PDCA is a dynamic cycle that can be deployed within each of the organization's processes, and to the system of processes as a whole. It is intimately associated with the planning, implementation, control and continual improvement of both product realization and other quality management system processes.

Maintaining and continually improving the process capability can be achieved by applying the PDCA concept at all levels within the organization. This applies equally to high-level strategic processes, such as quality management system planning, or management review, and to simple operational activities carried out as a part of product realization processes.

The Note in Clause 0.2 of ISO 9001:2000 explains that the PDCA cycle applies to processes as follows: