Although the term has been around since Plato's time, it is really only in the 20th century that it has acquired the significance it has today. The two most important names that have come to be associated with this concept are Jean Baudrillard and Gilles Deleuze.
Baudrillard tends to use the words simulation and simulacrum interchangeably, and offers not so much a new theory of the simulacrum as a new history of the present viewed through the conceptual lens of the simulacrum. For Baudrillard, the simulacrum is essentially the copy of a copy, that is to say, the copy of something that is not itself an original, and is hence an utterly degraded form. At its limit, as in certain accounts of postmodernism, the simulacrum is used to deny the possibility of anything being the singular source or origin of either an idea or a thing. On this view of things, anything deemed to be an original idea or object is in fact a mirage, an optical illusion of the same order as back-projection in cinema. Another way of putting this would be to say that a simulacrum is only ever an effect and never a cause.
In contrast, Deleuze uses the concept of the simulacrum against that of the simulation, to create an immanent theory of representation. The simulacrum is not a concept Deleuze wrote much about, indeed he only addresses it directly in two essays (on Plato and Lucretius) attached as appendices to Logique du Sens (1969), translated as Logic of Sense (1990), but his contribution has been decisive. His argument is that the copy or simulation is an image with resemblance (by definition, the copy resembles the thing copied), whereas the simulacrum is an image without resemblance (man is made in the image of God, according to catechism, but since the fall—i.e. because of his sin—he no longer resembles Him). The simulacrum is not just a degraded copy, Deleuze argues, it has its own positive power, which interrupts the relation between original and copy. His example of this is Pop Art, which in his view pushed the copy so far it became a simulacrum, an image without resemblance (e.g. Andy Warhol's famous Campbell's Soup prints).