I somewhat understand that the price of stocks is determined by the market, but who exactly changes those prices that we see listed on Yahoo! Finance for example?
If I own a share in a company listed in NASDAQ, does NASDAQ update the price based on how much people are willing to buy it at or sell it at? How does NASDAQ know what price I'm willing to sell or buy at?
The "price" is the price of the last transaction that actually took place.
According to Motley Fool wiki:
A stock price is determined by what was last paid for it. During market hours (usually weekdays from 9:30AM-4:00PM eastern), a heavily traded issue will see its price change several times per second. A stock's price is, for many purposes, considered unchanged outside of market hours.
Roughly speaking, a transaction is executed when an offer to buy matches an offer to sell.
These offers are listed in the Order Book for a stock (Example: GOOG at Yahoo Finance). This is actively updated during trading hours.
This lists all the currently active buy ("bid") and sell ("ask") orders for a stock, and looks like this:
You'll notice that the stock price (again, the last sale price) will (usually*) be between the highest bid and the lowest ask price.
* Exception: When all the buy or sell prices have moved down or up, but no trades have executed yet.
