When you press buy or sell, you send an order to your broker. The type of order controls how and at what price it fills.
Market order
Buy or sell right now at the best available price. It fills quickly, but in a thin or fast-moving stock the fill can be worse than the last price you saw.
Limit order
You name the price. A buy limit at $50 only fills at $50 or lower. A sell limit only fills at your price or higher. You control the price, but the order may never fill.
Stop-loss order
A stop-loss becomes a market order once the price touches your trigger. It is meant to get you out of a falling position. Pick a trigger that would show your idea was wrong, not a random percentage. A stop set too tight gets hit by normal daily swings.
The catch with stops
A stop does not guarantee your price. If a stock gaps down overnight, the stop triggers and fills at whatever price is available, which can be well below the trigger.
Stop-limit
A stop-limit turns into a limit order, not a market order, when the trigger hits. You control the worst price, but the order may not fill at all in a fast drop.
Education only, not investment advice. Trading involves real risk of loss, including loss of everything you put in. See the full disclaimer.