A limit order is used when a trader believes there will be a dip in the market and then a return of the pricing trend. ie If the price of AUDUSD is 0.76000, a trader can put a buy limit at 0.759950 and they will be entered into a buy trade at a better price than the current market price when it is executed.
A stop order is used when a trader wants to lock in a future price, in the same direction as the current trend. ie if the price of AUDUSD is 0.76000 a trader can lock in a buy stop trade at 0.76100 and when the price hits this level, they will be entered into their buy trade.