DEVS is a modeling formalism. Just as arithmetic underlies addition, multiplication, and other calculation, so DEVS underlies simulation of discrete event models.
The analogy between DEVS simulation and calculation is spelled out more fully in the DEVS Calculator Analogy.
DEVS stands for Discrete Event System Specification and is a way of specifying a subset from a wider set of dynamic input/output systems. DEVS models can represent a wide variety of discrete event formalisms, such as Petri Nets, and continuous formalisms, that were not developed from a systems-theory perspective.