A market order is an order that is executed immediately at current market prices. Traders use this order type when they are in need for an urgent execution.
Limit orders are used to specify a maximum or minimum price that a trader is willing to buy or sell at. Traders use this order type to minimise their trading cost, however they are sacrificing guaranteed execution. This is because there is a chance the order may not be executed if it is placed deep out of the market.