Timoteo is mostly right. A merchant buys and sells goods, but this term is a little old-fashioned. An entrepreneur is a businessman/woman who has taken initiative to start their own business, or has given new ideas to an existing business and reshaped how a company works. Entrepreneurs are proactive people who take risks in business. (Not always the same as a businessman) A businessman is a very generic term for anyone who works in the corporate world. If you are confused on what to call someone, use businessman (or businesswoman!)

A merchant is one who buys and sells good. A trader is one who deals in stock and securities. A businessman is one who works in business. There are little differences when it comes to formal definition, but they're all pretty much the same. They're synonymous to one another and have one thing in common - profit.