A formal, graphical, executable technique for the specification and analysis of concurrent, discrete-event dynamic systems; a technique undergoing standardisation. More on Petri nets...
A Petri net may be executed and the dynamic behaviour observed graphically. Petri net practitioners regard this as a key strength of the Petri net technique, both as a rich feedback mechanism during model construction and as an aid in communicating the behaviour of the model to other practioners and lay-persons.
Software tools exist which automate execution.