What is a market order?

Market orders to buy or sell securities will be executed immediately at the current market available price. 

A market order guarantees a fill or execution, but unlike a limit order, a market order provides no price protection. The price at which you actually execute (or fill) the trade can deviate from the last traded price, ending up with a price higher or lower than the market value you have previously seen due to the high volatility of the market or low trading volume of certain securities.