From my experience in buying/selling a very few contracts on the board.

If corn is 2.00 and I think it is going up I want to own corn.  I tell the broker I want to buy corn for 2.00.  He ask how badly do I want to buy corn?  Meaning.  It is now 2.00, but when I get off the phone with you it might be 2.01, because all the 2.00 is gone.  Do I want to buy corn at 2.01? Yes or no.  If I say yes and hang up the phone he will try and buy corn at 2.01.  Well, more people than me want corn.  Some people have told their brokers to buy corn for them for up to 2.05.  10 people will buy corn now at 2.05.  It is simply a biding process like an auction.  How bad do you or anybody else want to own or sell corn? 

Of course some people think corn is going down.  So they are willing to sell you corn at 2.00. 

The brokers are on the floor calling out to buy or sell grain at certain prices.  They hold their hand with their palms facing to themselves, this tells people they want to buy.  The sellers hold their palms away from their body telling they want to sell.  The "pit" also has steps.  It is built in a circle I think.  Depending on which step you are standing represents a price also I think.  I do not know how closely they watch which step they stand on.  It is a little confusing to the visitor watching the action with no on the spot explanation. 

I no doubt used all the incorrect terms is describing the action on the floor and people will blow me out of the water telling me how stupid I am about my explanation. 

But that is what I know IMHHHHO.