A stop order means that your order becomes active when a certain price is reached. Your order will be filled, but not necessarily at that exact price. (Also, a market order is an order to buy or sell at whatever the current market price is.)
A limit order means your order will be filled at a particular price only.
So if I own XYZ company and want to sell if the price goes to $50/share, I could put in a limit order for $50/share. But even if the price goes to $50, my order might not be filled because other people could be in line ahead of me.
If I own XYZ company and want to sell if the price ever gets to $50/share, I could put in a stop order. If the price gets to $50, my order becomes a market order. My order could be filled at a price somewhere above or below $50.