What is the difference between a market order and a limit order when buying a stock?

A market order buys or sells immediately at the best available price, so it fills fast but you don't control the exact price you pay. In fast-moving or thinly traded stocks, the fill can come in worse than the price you saw. A limit order sets the maximum you'll pay to buy (or minimum you'll accept to sell); it only fills at your price or better, but it may not fill at all if the stock never reaches your limit. For large, liquid ETFs and stocks a market order during regular hours is usually fine. For low-volume securities, near the open or close, or in volatile conditions, use a limit order to avoid a nasty surprise. New investors making small, routine purchases of broad index funds rarely need anything more than a simple market order.