With regular buy or sell orders you set the rate in which you wish to buy or sell and only when a matching offer is found at the same rate (or above if buying; below if selling) the deal is carried out.
Buy at market price - When you choose the option to buy at market price, you enter the amount you wish to spend and the system automatically gives you the amount you can purchase immediately for the entire sum, according to sell offers available at the time.
Sell at market price - When you choose the option to sell at market price, you enter the amount you wish to sell and the system automatically gives the highest possible price you can receive immediately for the entire lot, according to buy offers available at the time.