What other services besides buying or selling stocks does a stock broker perform?
In addition to trading (buying and selling stocks) for their clients, stock brokers may also offer advice to their clients on which stocks, mutual funds, etc. to buy. Stockbrokers also sometimes or exclusively trade on their own behalf, speculating that a stock will increase or decline in price. In this case they are considered simply traders, since they are not brokering for anyone else, but trading on their own behalf.