A stock broker or stockbroker is a regulated professional broker

Broker

A broker is a party that arranges transactions between a buyer and a seller, and gets a commission when the deal is executed. A broker who also acts as a seller or as a buyer becomes a principal party to the deal...

 who buys and sells share

Share (finance)

A joint stock company divides its capital into units of equal denomination. Each unit is called a share. These units are offered for sale to raise capital. This is termed as issuing shares. A person who buys share/shares of the company is called a shareholder, and by acquiring share or shares in...

s and other securities

Security (finance)

A security is generally a fungible, negotiable financial instrument representing financial value. Securities are broadly categorized into:* debt securities ,* equity securities, e.g., common stocks; and,...

 through market maker

Market maker

A market maker is a company, or an individual, that quotes both a buy and a sell price in a financial instrument or commodity held in inventory, hoping to make a profit on the bid-offer spread, or turn. From a market microstructure theory standpoint, market makers are net sellers of an option to be...

s or Agency Only Firms on behalf of investor

Investor

An investor is a party that makes an investment into one or more categories of assets --- equity, debt securities, real estate, currency, commodity, derivatives such as put and call options, etc...

s. A broker may be employed by a brokerage firm

Brokerage firm

A brokerage firm, or simply brokerage or broker in context, is a financial institution that facilitates the buying and selling of financial derivatives between a buyer and a seller...

.
A transaction on a stock exchange

Stock exchange

A stock exchange is an entity that provides services for stock brokers and traders to trade stocks, bonds, and other securities. Stock exchanges also provide facilities for issue and redemption of securities and other financial instruments, and capital events including the payment of income and...

 must be made between two members of the exchange—an ordinary person may not walk into the New York Stock Exchange

New York Stock Exchange

The New York Stock Exchange is a stock exchange located at 11 Wall Street in Lower Manhattan, New York City, USA. It is by far the world's largest stock exchange by market capitalization of its listed companies at 13.39 trillion as of Dec 2010...

 (for example), and ask to trade stock. Such an exchange must be done through a broker.

There are three types of stockbroking service.