A limit order gives you certainty about execution price but not about time.
A market order gives you certainty about execution time but not about price.
A market order deals with the execution of the order. In other words, the price of the asset is secondary to the speed of completing the trade. Limit orders, on the other hand, deal primarily with the price. So, if the security's value is currently resting outside of the parameters set in the limit order, the transaction does not occur.
Market Order
Easier to set up as no price is specified
Will almost always be filled immediately
Limit Order
You must specify price at which order will trigger
May not get filled if limit order price is not met by market