Discrete Event Simulation (DES) is a technique of modelling where all changes in the simulation model are represented by events (nodes) that occur when particular circumstances are met (edges). These models usually have a graphical representation of nodes and edges with all relations between them.
All changes in the model are visible in the graph so it is easy to understand how the model works. Models are usually stochastic, so the occurrence of particular event is described by given probability distribution.
A simulation may run many times to test different scenarios and define e.g. a probability of not meeting the deadline of the project (in a DES PERT model) and indicate the most common situations when a deadline is not met.
As the simulated project keeps going and following parts of it are finished, the model is updated and stochastic data is replaced by the real time information, so the outcome is becoming more and more precise, making it a great tool for a project risk management.
DES models are very flexible, they could be used to simulate complex working production systems or a new queuing method in the airport.