Motivations

Why are we talking about UML state machines?
Because of SCXML.
What is SCXML?
State Chart XML = SCXML (currently a W3C Note)
SCXML preview available in the sandbox section of Apache Jakarta Commons

I have been involved in developing the SCXML interpreter and as a consequence I have learned a thing about UML state machines and "Harel Statecharts" in general. The goal is to share the knowledge with you before I manage to forget most of it.