All that money you've lost _ where did it go?

We all notice the results of the recent decline of the Stock market. We hear about how many years this means that are lost for those planning to retire based on those results.
What exactly does this all mean? Was it actually your money to begin with? Did it come from your pocketbook, so to speak?
What defines money that we have actually lost due to the collapsing stock market as everyone starts to sell? This article illustrates this and other points regarding the definitions of loss.