Image via WikipediaLimit Order - An order to buy or sell currency at a certain limit is called Limit Order. When you buy, your order is carried out when the market reached down your limit order price. When you sell, your order is carried out when the market reaches up your limit order price. You can use it to buy currency below the market price or sell currency above the market price. There’s no decrease with limit orders.
Market Order

Stop Order - An order to buy above the market or to sell below the market. It’s usually used as a stop-loss order to diminish losses if the market behaves opposite to what the broker supposed. A stop-loss order lets sell the currency if the market goes below the point appointed by the broker. In Forex market there are four various types of stop-orders.

Market Order - An order to buy or sell at the running market price. Market orders should be used very carefully as in fast-changing markets there’s sometimes a disparity between the price when the market order is given and the actual price of the deal. This occurs because of market decrease. It can lead to a loss or gain of several pips. Market orders can be used to enter or exit a trade.

OCO Order (One Cancels the Other) - An order that used in case if one simultaneously places a limit order and a stop-loss order. If either order is carried out the other is abrogated which lets the broker to make a deal without supervising the market. Once the market reaches up the level of the limit order, the currency is sold at a profit but when he market falls, the stop-loss order is used.