Review: What is a State Diagram?
A state transition diagram is a pictorial representation of a finite state machine. Usually, the states are represented as circles and the transitions are represented as directed line segments.
A finite state machine is a function which maps an ordered sequence of input events into a corresponding sequence of (sets of) output events*.
* http://wombat.doc.ic.ac.uk/foldoc/foldoc.cgi?finite+state+machine