Market order:

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 order:
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.