This web site uses several technical terms that can easily be confused. Here we try to clarify some of them. If you see anything missing, let us know!

Model A mathematical or logical representation of an entity, phenomenon or process, or system of such. Often, a model is formulated as a set of differential equations, but many other modelling techniques exist.
Solver A program, or more generally an algorithm, that solves the model equations. For differential equations, this is an integration algorithm.
Simulation The act of using a solver to compute how one or more models behave under specified conditions.
Modelling tool Software that can be used to create a model. Most modelling tools include a solver, so that they become modelling and simulation tools.
Simulation tool Software that can be used to perform a simulation.