[Sniper]

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could you please explain to me what a 'trader' is?
how is that different from a stock broker?

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A stock broker is a commissioned salesman that takes the research produced by a research analyst and sells you on the reasons to buy a stock. When you make a trade the broker sends the trade to a trader for actual execution. The broker makes his money by convincing you to make trades, and sometimes (ala Boiler Room - definately recommended movie) getting you into questionable IPOs, or other low quality stocks that they may be compensated for dumping on you.

A Trader, actually executes a stock transaction in the market. They may be doing this on behalf of a firm, acting as a market maker/specialist, managing retail (customer) orders, or for their own account.