If you visit the SEC (Securities and Exchange Commission) website the answer to this question is:

The mission of the U.S. Securities and Exchange Commission is to protect investors, maintain fair, orderly, and efficient markets, and facilitate capital formation.

Sounds fairly straightforward right?? It's far more complicated than it looks.

The SEC is responsible for oversight of every public traded company, as well as the New York Stock Exchange and the other Exchanges (i.e. Boston Stock Exchange, Chicago Merchantile Exchange, etc.) and control over all Broker Dealers, financial filings for all companies including quarterly filings, annual filings, dividend claims, etc.

From an investors standpoint the SEC is somewhat of a police officer who monitors all activities in companies that we might invest in and makes sure they're not taking us for a ride.

The New York Board of Trade is a wholly owned subsidiary of Intercontinental Trade and is a commodities market trading place physically located in New York City.

In 1870 the name was the New York Cotton Exchange, however in 1998 the New York Board of Trade became the 'parent' company of not only the New York Cotton Exchange but also of the New York Coffee Sugar and Cocoa Exchange (which was founded in 1882.

In 2007 they completed a transaction which made them a unit of the Intercontinental Exchange (ICE).

Each individual company that trades on the exchange must send its own independent brokers. Therefore, a few employees on the floor of the exchange represent a big corporation and the exchange employees record the transactions and have nothing to do with the actual trade.

The NYBOT Trades the following:
Cocoa
Coffee
Cotton
Ethanol
Wood pulp
Domestic Sugar
World Sugar
Frozen Concentrate Orange Juice

NASDAQ is an acronym for National Association of Securities Dealers Automated Quotations. NASDAQ is the largest screen based trading market in the United States. Currently, more than 3,000 companies trade their shares there.

You will find some familiar names traded on the NASDAQ such as Apple Computer (symbol AAPL), Adobe Systems (ADBE), Microsoft Corportation (MSFT), Google (GOOG) and Yahoo (YHOO) are the ones that you will immediately recognize.

The Borsa Italiana S.p.A. is based in Milan. It is Italy's main stock exchange. Borsa Italiana was made private in 1997, and was bought out by the London Stock Exchange, plc, the same year. As of 2005, Borsa Italiana's list of companies was valued at over $890 billion.

Borsa Italiana has the responsibility of managing Italy's derivatives markets (IDEM and MIF) as well as its fixed income market (MOT). On the Electronic Government Bond and Securities Market (MOT) buy and sell contracts are traded on nonconvertible bonds and government securities. However, the EuroMOT trades Eurobonds, bonds from foreign issuers and asset-backed securities.

Borsa Italiana represents, manages, and organizes Italy's stock market with the membership of nearly 130 brokers (domestic and international) who function both in Italy and abroad through remote membership. They use a completely electronic trading system for the actual-time implementation of trades. It also performs organizational, commercial, and promotional activities geared toward the development of high value-added services for the financial community.

Borsa Italiana is divided into five parts. The MTA (electronic share market) trades Italian shares, convertible bonds, and warrants. The covered warrants market is an example of what is represented by the electronic share market. The Segment for High Requirement Shares (STAR) market is within the MTA and includes companies capitalized from 40-100 million Euro which are already both listed and traded in other traditional sectors. Nuovo Mercato is set aside for innovation-driven companies. The Mercato Ristretto trades stocks, bonds, warrants, and options that are not admitted on the official exchange. Premi Market is self-explanatory: premium contracts on stock exchange products. There is also an after-hours market which enables trading of financial instruments after the typical daytime session closes.

The London Stock Exchange, also known as the LSE, is located in London, England, United Kingdom. It was founded in 1801, and is one of the largest stock exchanges in the world. It has both overseas listings as well as British companies. The London Stock Exchange is part of the London Stock Exchange Group, plc.

The LSE is comprised of the Main Market, Alternative Investments Market (also called AIM), EDX London (which handles offshoots), Professional Securities Market, Specialist Fund Market, and the independent FTSE group, which maintains a series of indices for the measurement of the LSE. This includes the FTSE 100 Index, 250 Index, and 350 Index. There are about 2,800 companies listed with the LSE as of March, 2008. The LSE makes it possible for companies (from global giants to small enterprises) around the world to raise the capital they need in order to grow. Once the company has been admitted into trading, the LSE assists them in maximizing the value of said listing.

In December 2005, the LSE rejected a buy-out offer of £1.6 billion from Macquarie Bank, touting the offer as "derisory." This sentiment was echoed by shareholders in the exchange. Shortly after this rejection, the LSE received another offer, this time from NASDAQ. NASDAQ's offer of £2.4 billion was also rejected. After several buy-outs of other stock exchanges on the part of NASDAQ in an attempt to force a hostile takeover on the LSE, NASDAQ made a higher per share buy-out offer to the LSE. This offer was also refused. Subsequently, NASDAQ retracted its offer and has since given up its pursuit of the LSE.

Note: the LSE had a 40 minute outage/downtime in November, 2007 due to connectivity issues.