A trader is a person who (buys or sells) securities or derivatives on any market with a view to profiting from that trade via the trader's (usually) short-term strategy plan. Investors, by contrast, generally aim for long-term profits by buying and holding stock in companies they are familiar with.
Most traders execute their trades either directly from an exchange via a floor broker or market maker, or in the secondary market, also called the over-the-counter (OTC) market, from other trading or investing firms. A Floor Trader is an exchange member who buys and sells directly from the exchange on their own account.
Traders, unlike investors, are usually not concerned with the underlying asset they are trading - it is simply "in the right place at the right time". They trade to a plan, restrict their potential losses and usually profit by following clear trends in investment markets.