This looks at various ways that computer simulations can differ not in terms of their detailed mechanisms but in terms of its broader purpose, structure, ontology (what is represented), and approach to implementation. It starts with some different roles of people that may be concerned with a simulation and goes on to look at some of the different contexts within which a simulation is set (thus implying its use or purpose). It then looks at the kinds of system that might be simulated. Shifting to the modelling process, it looks at the role of the individuals within the simulations, the interactions between individuals, and the environment that they are embedded within. It then discusses the factors to consider in choosing a kind of model and some of the approaches to implementing it.