A stop order is an instruction to buy or sell that remains dormant until the market trades at a specified trigger level, called the stop price, and its most common form is described in the main entry on the stop-loss order. Once triggered, a plain stop order turns into a market order and is executed at the best price then available, which may differ from the stop price.
The term stop order is the broader of the two. A sell stop is placed below the current price and is normally used to limit the loss on a holding, which is the stop-loss case. A buy stop is placed above the current price. It is used to cap the loss on a short position, or to enter a trade when the price rises through a level, a technique used in breakout trading.
Because execution is at the market, a stop order can fill well away from the trigger in a fast or gapping market, an effect known as slippage. A stop-limit order addresses this by converting into a limit order instead, at the cost of possibly not trading at all.