Having a stop-loss order on each position in your portfolio is, in my opinion, vital to responsible trading. Some traders dismiss stops as being unnecessary. Perhaps they think that a mental stop will be sufficient (a mental stop is a pledge to yourself to get out of a position if you think it's going wrong). Other more conspiracy-minded individuals believe that market makers gun for stops, taking small traders out of positions with some magical knowledge of their stop-loss levels.
Both of these notions are silly. Having a mental stop in place of a real stop is a way of cheating yourself out of trading with integrity, and in a market as big as ours, the idea that market makers have nothing better to do than omnipotently push prices up and down in order to zap small retail traders out of their positions is absurd.
In case you are not acquainted with a stop-loss order, it simply is a standing order to execute a trade based on a certain condition. Typically this condition is one in which a price is violated to the upside (for short positions) or downside (for long positions). A stop-loss order may sit in your account for days or weeks without ever executing, but that's the entire idea: It is there just in case something happens, and it may never be executed.
When a stop-loss order is entered, it can be input as a market order or a limit order; the latter requires an execution to be at a certain price. Again, I offer no nuances here: Using ...