A market order is an order to buy or sell a stock at the current market price. Unless you specify otherwise, your broker will enter your order as a market order. The advantage of a market order is that as long as there are willing buyers and sellers, you are almost always guaranteed your order will be executed. The disadvantage is the price you pay when your order is executed may not be the price you expected.

A "market maker" is a firm that stands ready to buy or sell a stock at publicly quoted prices. 

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A market index tracks the performance of a specific "basket" of stocks that represent a particular market or economic sector. U.S. examples include the Dow Jones Industrial Average, an index of 30 "blue chip" U.S. company stocks, the Standard and Poor's 500 Index, and the Wilshire 5000 Index, which includes most publicly traded U.S. stocks.

The amount of goods and services that can be purchased by a given unit of currency, taking into account the effect of inflation.

Revenue minus cost; money made on a transaction.