There are two generally used execution types offered by Forex Brokers. There is Instant Execution or Market Execution and both of them have their advantages as well as their disadvantages.
Instant execution refers to the required price quoted by the trader. In other words, if the trader wishes to buy 1 lot EUR/USD at 1.3827 then he expects his order to be executed on that rate instantly. If the price has changed, then the trader will receive a re-quote on his request with the new rate. In such case, the order will not be executed unless the trader confirms that he/she accepts the new rate quoted by the Forex Broker. Instant execution offers more control to the trader as he/she will be able to confirm the rate that he/she wishes to trade without any sort of slippage. The bad news is that sometimes there is a delay to execute such a trade and thus this may not be suitable for traders that use scalping strategies or when there are news releases because of the delay.