from: Undertaking systematic reviews of research on effectiveness: CRD's guidance for those carrying out or commissioning reviews. CRD Report 4 (2nd edition). March 2001. http://www.york.ac.uk/inst/crd/report4.htm
It is often applied in the healthcare context, but is also sometimes used in educational and social policy research.
A systematic review is a research method, just as interviews or surveys are research methods. The key difference is whether you are collecting and analysing your own primary data, or performing a review of already published research and grey literature.