A stock broker is a person whose business is buying and selling of companies stocks and shares.
They are specifically trained to buy and sell of stocks and shares for an investor that is their client at a fixed commission. This commissions depends on the stock broking firms they belong to. Investment commission is a certain percentange of the invested capital.
Unlike ebay or other market places where people who have products and people who wants to buy the the products assemble to exchange products for cash. The stock market does not assemble buyers and sellers directly; things are done most of the time through agents known as stock brokers.
They execute the mandate of their clients, they can also help you in investment, education, advice and decisions with their analysis skills, they can also be empowered to manage your investment portfolio.
Now a days with the advent of internet there are lot of online stock brokers who are dealing members of the stock exchange and as members of the exchange they are traders. They buy and sell equities on behalf of their clients as investors.
So it is the business of buying and selling shares on behalf of investors.
But before you can start investing you need to open a brokerage account which allows you to start investing by buying, selling of stocks,bonds, mutual funds. You also have the option of setting up automatic monthly withdrawals, where you can instruct that a specified amount of money be tranfered from your checking account to your brokerage account every month.