This video, created by Cochrane, gives a basic explanation of what a systematic review is:
http://www.cochrane.org/news/what-are-systematic-reviews
Manuals.
Cochrane Handbook for Systematic Reviews of Interventions - online or print. This is likely the single best source for systematic reviews.
Centre for Reviews and Dissemination (CRD), University of York, UK - see the section called "Our Guidance"
JBI Reviewers’ Manual
PRISMA Transparent Reporting of Systematic Reviews and Meta-analyses - http://www.prisma-statement.org/
Cochrane Methodological Expectations of Cochrane Intervention Reviews (MECIR) - http://editorial-unit.cochrane.org/mecir
Institute of Medicine Standards - http://www.iom.edu/Reports/2011/Finding-What-Works-in-Health-Care-Standards-for-Systematic-Reviews/Standards.aspx