A stock exchange does not own shares. Instead, it acts as a sort of high-tech flea market where buyers connect with sellers. Every public stock trades on one of several possible exchanges such as the New York Stock Exchange (NYSE) or American Stock Exchange (AMEX). Although you will most likely trade stocks through a broker, it is important to understand the relationship between exchanges and companies and the ways in which the requirements of different exchanges provide protection to investors.

How Does It All Start?
The primary function of an exchange is to provide liquidity; in other words, to give sellers a place to "liquidate" their share holdings.

Stocks first become available on an exchange after a company conducts its initial public offering (IPO). In an IPO, a company sells shares to an initial set of public shareholders (the primary market). After the IPO "floats" shares into the hands of public shareholders, these shares can be sold and purchased on an exchange (the secondary market).

The exchange tracks the flow of orders for each stock, and this flow of supply and demand sets the price of the stock. Depending on the type of brokerage account you have, you may be able to view this flow of price action. For example, if you see that the "bid price" on a stock is $40, this means somebody is telling the exchange that he or she is willing to buy the stock for $40. At the same time you might see that the "ask price" is $41, which means somebody else is willing to sell the stock for $41. The difference between the two is the bid-ask spread.

Auction Exchanges - NYSE and Amex
The NYSE and AMEX are both primarily auction-based, which means specialists are physically present on the exchanges’ trading floors. Each specialist "specializes" in a particular stock, buying and selling the stock in a verbal auction. These specialists are under competitive threat by electronic-only exchanges that claim to be more efficient (that is, execute faster trades and exhibit smaller bid-ask spreads) by eliminating human intermediaries.

The NYSE is the largest and most prestigious exchange. Collectively, as of December 31, 2007, its listed companies represent about $30.5 trillion in market capitalization.

Listing on the NYSE affords companies great credibility because they must meet initial listing requirements and also comply annually with maintenance requirements. For example, to remain listed, NYSE companies must keep their price above $1 and their market capitalization (number of shares x price) above $50 million.

Furthermore, investors trading on the NYSE benefit from a set of minimum protections. Among several of the requirements that the NYSE has enacted, the following two are especially significant:

1. Companies must get shareholder approval for any equity incentive plan (for example, stock option plan or restricted stock plan). In the past, companies were allowed to sidestep shareholder approval if an equity incentive plan met certain criteria; this, however, prevented shareholders from knowing how many stock options were available for future grant.
   
2. A majority of the members of the board of directors must be independent. However, each company has some discretion over the definition of "independent", which has caused controversy. Furthermore, the compensation committee must be entirely composed of independent directors, and the audit committee must include at least one person who possesses "accounting or financial expertise".

AMEX is a smaller but quite prestigious exchange. AMEX also has a history of innovating: it pioneered the concept of exchange-traded funds (ETFs) and it has the second largest options trading market.