A stop order sits dormant until the stop price is hit, then it converts into a market-order. Sell stops below the market are the usual stop-loss; buy stops above the market are used to enter on a breakout or to cover a short.
Because it becomes a market order, a stop guarantees an exit but not a price. See stop-limit-order for the alternative.
Example: you are long at $52 with a sell stop at $50. Price trades at $50 and the order fires; it fills at $49.95 in a normal market or well below in a gap.
Related: stop-loss, stop-limit-order, market-order, trailing-stop, slippage