I delivered a couple of month ago a Valtech training session about applying UML2 and SysML to model complex systems. Following this training session, I decided to write an article to provide an overall picture about this UML-derived modelling language designed for Systems Engneering.
This article is entitled "SysML Modelling Language explained" (PDF version), and covers SysML 1.2.
SysML is based on UML and involves modelling blocks instead of modelling classes, thus providing a vocabulary suitable for Systems Engineering. A block encompasses software, hardware, data, processes, personnel, and facilities.
SysML reuses a subset of UML2 (UML4SysML) and defines its own extensions. Therefore SysML includes nine diagrams instead of the thirteen diagrams from UML2, making it a smaller language that is easier to learn and practice.
SysML is maintained by the OMG and is still evolving (first released as a standard in 2006, version 1.2 has become available in June 2010). It's also supported by most UML modelling tool, such as SparxSystems Enterprise Architect (Ultimate version, or with SysML plugin).