CROS stands for "Portal on Collaboration inResearch and Methodology for Official Statistics". The CROS Portal is dedicated to the collaboration between researchers and Official Statisticians in Europe and beyond. It provides a working space and tools for dissemination and information exchange for statistical projects and methodological topics.
Read more about it on: http://ec.europa.eu/eurostat/cros/page/about-cros-portal
The help pages on CROS contain rich material on the material available on CROS and how you can use it.