Broker:

A broker is a party that mediates between a buyer and a seller.A broker who also acts as a seller or as a buyer becomes a principal party to the deal. Distinguish agent: one who acts on behalf of a principal. A brokerage or a brokerage firm is a business that acts as a broker. A sales person working for a securities or commodity brokerage firm is popularly (but incorrectly) called a "broker." A broker in that context is, strictly speaking, an exchange member who actually executes the purchase or sale order in the pit, on the exchange, as a service to the client of the firm for which that salesman works.
Broker-dealer is a compay or other organization that trades securities for its own account or on behalf of its customers.
When executing trade orders on behalf of a customer, the institution is said to be acting as a broker When executing trades for its own account, the institution is said to be acting as a "dealer." Securities bought from clients or other firms in the capacity of dealer may be sold to clients or other firms acting again in the capacity of dealer, or they may become a part of the firm's holdings.
Although many broker-dealers are "independent" firms solely involved in broker-dealer services, many others are business units or subsidiaries of commercial banks, Investment banks or investment companies