In short, an emulator is an app itself that emulates (presents to be) a specific hardware device–such as an iPhone.
In recent years a number of iOS emulators have popped up for both Mac and Windows. The sad thing about this is that you need an iOS device to run them. There are literally MILLIONS of apps and games that are designed to run on iOS.
But compared to the iPhone, Mac, and PC app offerings combined are slim.
