I am always confused between network simulator / network simulation) and network emulator / network emulation.
Below are what I understand so far.
Simulation is for analysis and study, while emulation is for usage as a substitute.
A simulator models but an emulator is one that replicates the usage as on the original device or system.
A network simulator would use mathematical models to simulate and show result to the application which is running on a computer, using a mathematical model with assumptions and/or actual calculation.
Network emulators take the next step on the contiuum from pure mathematics and software towards physical reality, it imitates the function with modifications to hardware, software, or network activity that allow it to accept the same data, execute the same programs, and achieve the same results as the imitated system.
You can read more about simulator and emulator terminology, and what network simulator, network emulator are , or short video of Steve Norry (Director of Technical Sales for Scalable Network Technologies) speaking on network emulation vs. simulation.