OMNeT++ is an open-source, component-based discrete event network simulation framework written in C++. It allows models to be built hierarchically from components (modules) that communicate through message passing. OMNeT++ provides a simulation kernel and class libraries, while models are developed by defining module structures in NED and programming module behavior in C++. It supports simulations of protocols, traffic, and distributed systems and is commonly used for evaluating network and hardware performance.