See Investopedia - What's the Difference Between a Stop and a Limit Order?:
A stop-limit order consists of two prices: a stop price and a limit
price. This order type can be used to activate a limit order to buy or
sell a security once a specific trigger price or stop has been met.
For example, imagine you purchase shares at $100 and expect the stock
to rise. You could place a stop-limit order to sell the shares if your
forecast was wrong. If you set the stop price at $90 and the limit
price as $90.50, the order will be activated if the stock trades at
$90 or worse. However, a limit order will be filled only if the limit
price you selected is available in the market. If the stock drops
overnight to $89 per share, that is below your stop price, so the
order will be activated, but it will not be filled immediately because
there are no buyers at your limit price of $90.50 per share. The stop
price and the limit price can be the same with this order type.
Stop Market Order
A stop market order will turn into a traditional market order once
your stop price is met or exceeded.
In the previous example, the stop-limit order was not filled at $90.50
because that price was not available in the market when the stock
dropped through the stop price overnight.
A stop market order is triggered once the price is equal to or beyond
$90 and in this example will be executed at $89 or the best
alternative price. In these two scenarios, the stop order is used to
control losses, and both orders are sometimes referred to as a "stop
loss" by traders.