A market order allows you to buy or sell an asset immediately at the current market price. Its advantage is an instant completion and its disadvantage is the fact that you cannot predict the exact price at which this order will be executed.
A limit order allows you to buy or sell an asset at a fixed price. Due to this, it will not be completed instantly but only when a counter order (or orders) appears for buying or selling funds at the price set by you. Moreover, the price specified in the order is removed from your trade balance and is reserved in the order.