First, I would suggest reading the
Wiki Page on PensionsSince occupational pensions are basically paid as an annuity, this implies there is some "investment to produce gains" on the part of the agency issuing the annuity contract. They take your money and guarantee an annual sum payback. Because this is generally an instrument of the insurance companies, you can bet they are using your money to make enough money to pay you back plus put something in their own pockets. What means they use for those investments, stocks, bonds, etc, would be up to each company. So in effect the company that has a pension contract is basically running a private mutual fund in order to generate the necessary $$$ to fund the contract.
This is really an over simplification, which is why I suggested reading the Wiki reference above.
As for how much of "the market" is funded by pensions, 401k's, and other retirement plans.... I don't really know.
I would guess that since something like 90% of the worlds wealth belongs to 1% of the population (my stats may be wrong there), the vast majority of "street paper" belongs to people/companies that have some type of arrangement with fund management firms that are not associated with the concept of "an employer". Granted, this would include annuities, stocks, bonds, etc... but not part of an employment package per-se.
Hope that helps....
MZr7
edit: BTW... I'm not an expert in this at all, but I do manage the majority of my own retirement funds. (and I did once stay in a red roof inn) :)