What are the different stock exchanges or markets?

First, what is a stock exchange or stock market you ask (also known as share markets)?  Basically, it is a company or organization that facilitates the buying and selling of stocks and other securites which is controlled by the US SEC.

 What are the different stock exchanges? According to the US securties and exchange commision:

A “national securities exchange” is a securities exchange that has registered with the SEC under Section 6 of the Securities Exchange Act of 1934.

There are currently ten securities exchanges registered with the SEC under Section 6(a) of the Exchange Act as
  national securities exchanges:

• American Stock Exchange
• Boston Stock Exchange
• Chicago Board Options Exchange
• Chicago Stock Exchange
• International Securities Exchange
• The Nasdaq Stock Market LLC
• National Stock Exchange (formerly the Cincinnati Stock Exchange)
• New York Stock Exchange
• NYSE Arca, Inc. (formerly the Pacific Exchange)
• Philadelphia Stock Exchange

Certain exchanges are also registered with the SEC through a notice filing under Section 6(g) of the Exchange Act for the purpose of trading security futures.

There are also two exchanges that the SEC has exempted from registration as national securities exchanges on the basis of a limited volume of transactions:

• Arizona Stock Exchange
• virt-x plc (formerly known as Tradepoint)

Now, why are there so many stock markets or exchanges ?  In essence, they track or trade different things in different ways, like in person or by electronic automaton. For example, the largest in the World is the NYSE. The DJIA or dow jones industrial average is an indicator and representative of the largest public companies financial health, also called your blue-chip companies.