This document defines computer simulation as imitating real things, states, or processes and key characteristics of selected physical or abstract systems. It discusses categories of simulation based on application area and functionality. Simulation is needed for complex models with nonlinear variable relationships or random variables. Simulations provide graphical screens, simulators, and 3D graphics. They have applications in advanced analytics, airports, manufacturing, space, healthcare, military, and mining. Advantages include studying systems without building them and performing "what if" analysis, while disadvantages include costs and difficulty interpreting results.