A stock exchange is an organization that provides a marketplace for either physical or virtual trading shares, bonds and warrants and other financial products where investors (represented by stock brokers) may buy and sell shares of a wide range of companies. A company will usually list its shares by meeting and maintaining the listing requirements of a particular stock exchange. In the United States, through the inter-market quotation system, stocks listed on one exchange can also be bought or sold on several other exchanges, including relatively new so-called ECNs (Electronic Communication Networks like Archipelago or Instinet).



In the USA stocks used to be broadly grouped into NYSE-listed and NASDAQ-listed stocks. Until a few years ago there was a law that NYSE listed stocks were not allowed to be listed on the NASDAQ or vice versa.

Many large non-U.S companies choose to list on a U.S. exchange as well as an exchange in their home country in order to broaden their investor base. These companies have then to ship a certain number of shares to a bank in the US (a certain percentage of their principal) and put it in the safe of the bank. Then the bank where they deposited the shares can issue a certain number of so-called American Depositary Shares, short ADS (singular). If someone buys now a certain number of ADSs the bank where the shares are deposited issues an American Depository Receipt (ADR) for the buyer of the ADSs.

Likewise, many large U.S. companies list themselves at foreign exchanges to raise capital abroad.

Although it makes sense for some companies to raise capital by offering stock on more than one exchange, a keen investor with access to information about such discrepancies could invest in expectation of their eventual convergence, known as an arbitrage trade. In today's era of electronic trading, these discrepancies, if they exist, are both shorter-lived and more quickly acted upon. As such, arbitrage opportunities disappear quickly due to the efficient nature of the market.

The price of a stock fluctuates fundamentally due to the theory of supply and demand. Like all commodities in the market, the price of a stock is directly proportional to the demand. However, there are many factors on the basis of which the demand for a particular stock may increase or decrease. These factors are studied using methods of fundamental analysis and technical analysis to predict the changes in the stock price. A recent study shows that customer satisfaction, as measured by the American Customer Satisfaction Index (ACSI), is significantly correlated to the stock market value. Stock price is also changed based on the forecast for the company and whether their profits are expected to increase or decrease.