The term simulacrum is most associated, in this day and age, with
French sociologist Jean Baudrillard. It was first introduced in his essay, "The Precession of Simulacra" (which is the first chapter of the book
Simulations, later re-published in the now popular
Simulacra and Simulation).
In this essay, Baudrillard describes the procession, the action of the process, of how the simulacrum ultimately became more than a mere simulation and replaced reality. In other words, the simulacrum has become the hyperreal.
The difference between the simulation and the simulacrum, is the former is distinguishable from the real, and the latter has come to be indistinguishable from the real, or has - as Baudrillard argues - replaced it as the hyperreal.
An example would be to take the common images of Abercrombie and Fitch models as simulations of young people of America. Where as before the fashion of the company was a copy of the beach attire of teenagers and those in their 20s. The simulation now no longer exists as a copy, but rather people now copy that image as reality. So reality has turned itself inwards, and has copied the image, rather than the image copying reality. The simulacrum has been given itself a life. Other examples to be cited could be Betty Boop and pin-up models, TV shows like Beverly Hills 90210 and high schoolers, etc.
Reference
Baudrillard, Jean. "The Procession of the Simulacra." Simulations. New York: Semiotext(e), 1983.