A Simulacra, in it’s most pure form is a “copy of a copy”. However, it is slightly more than that, often a simulacrum would be a copy that has changed so much from it’s original that it can no longer be said to be a copy. Or, a copy of something that no longer exists, or has been forgotten, so only the copy remains.
For Example, a piece of art painted from a photo could be called a simulacrum, especially if the original object of the photograph no longer exists.

Which leads me onto…

Are humans simulacra? And If so what of?

I think we are simulacra of everything we experience, everything that shapes our reality. As a basic example, someone who obsessively reads newspaper scare stories ends up being controlled by those stories, their whole life becomes one giant scare story. Of course, in reality, we draw from far more sources to create our world view, but is our interpretation of events just limited by what we have experienced, if so are we just copying what we have alread seen in our own fashion?

Many would believe we are a simulacra of god:
In Genesis it says: “God created humankind in his own image”
But what does that mean? Are we a “less powerful/clever/aware” version of God? Perhaps we are similar to God in a cosmetic sense? Perhaps there is a little bit of God inside each of us? Perhaps God made us to act in his place? Or of course, perhaps there was never a God, and we are a simulacra with no original.

Maybe we are also simulacra of ourselves, there is always “yourself” behind every action you do, observing and thinking almost independently of your other “self”. Perhaps both of these “yourselves” are simulacra of each other, if that is the case, where do “you” lie, somewhere in between?

That is my deifnition of Simulacra.

“We can never finally know. I simply believe that some part of the human self or soul is not subject to the laws of time and space.” - C.G. Jung