You give your broker a sell order and he/she goes out and finds the best deal he/she can. If you specify a price, he/she will sell only at that price or above. If you want to sell regardless, he'll sell at whatever he can get. It's a pool where thousands (millions?) of individuals and companies/corporations, etc. have stocks floating around in the pool. The value is determined by what someone is willing to pay at any given moment.

Your broker will look for "buy" orders from other brokers and will sell at the best value he/she can. If you specify to only sell at $x.xx or above, the broker will only sell your stock if he can find someone willing to pay that amount. Or he puts your sell order out there and the "buyers" look at it and decide if they want to pay what you're asking.

Does that make sense?

This will explain it to you:
http://www.youngmoney.com/investing/sharebuilder/goals/031022_01

Or this looks like fun. I might just might play it for fun! http://www.howthemarketworks.com/

I got out of the stock market a couple of days ago and put all my monies into guaranteed because I was sick of being sick at the sight of my retirement money going south. I can get back in whenever I feel the economy is gearing up again. I'd love playing it if I were younger.