A stock market is a public market for the trading of company stock and derivatives at an agreed price; these are securities listed on a stock exchange as well as those only traded privately.

The size of the world stock market was estimated at about $36.6 trillion US at the beginning of October 2008 . The total world derivatives market has been estimated at about $791 trillion face or nominal value, 11 times the size of the entire world economy. The value of the derivatives market, because it is stated in terms of notional values, cannot be directly compared to a stock or a fixed income security, which traditionally refers to an actual value. Moreover, the vast majority of derivatives 'cancel' each other out (i.e., a derivative 'bet' on an event occurring is offset by a comparable derivative 'bet' on the event not occurring.). Many such relatively illiquid securities are valued as marked to model, rather than an actual market price.)

The stocks are listed and traded on stock exchanges which are entities a corporation or mutual organization specialized in the business of bringing buyers and sellers of the organizations to a listing of stocks and securities together. The stock market in the United States includes the trading of all securities listed on the NYSE, the NASDAQ, the Amex, as well as on the many regional exchanges, e.g. OTCBB and Pink Sheets. European examples of stock exchanges include the London Stock Exchange, the Deutsche Börse and the Paris Bourse, now part of Euronext.

Trading

Participants in the stock market range from small individual stock investors to large hedge fund traders, who can be based anywhere. Their orders usually end up with a professional at a stock exchange, who executes the order.

Some exchanges are physical locations where transactions are carried out on a trading floor, by a method known as open outcry. This type of auction is used in stock exchanges and commodity exchanges where traders may enter "verbal" bids and offers simultaneously. The other type of stock exchange is a virtual kind, composed of a network of computers where trades are made electronically via traders.

Actual trades are based on an auction market model where a potential buyer bids a specific price for a stock and a potential seller asks a specific price for the stock. (Buying or selling at market means you will accept any ask price or bid price for the stock, respectively.) When the bid and ask prices match, a sale takes place on a first come first served basis if there are multiple bidders or askers at a given price.

The purpose of a stock exchange is to facilitate the exchange of securities between buyers and sellers, thus providing a marketplace (virtual or real). The exchanges provide real-time trading information on the listed securities, facilitating price discovery.