Market order is a request by a trader to execute a transaction as quickly as possible against the best market price. Market order guarantees the order execution but does not guarantee the price. That’s why this type of order is used when the speed is more important than the price of execution.
Market order implies the willingness of a trader to buy at Ask price or to sell at Bid price. However, since Market order does not guarantee the price, a difference between the expected price of a trade and the price at which it was actually executed may occur. This difference is called slippage.
Slippage can happen due to increased volatility or if a large order is executed and there is not enough liquidity for the price to stay the same.
Those who prefer having an order executed at a specific price (or better) should use Limit order. Placing Limit order, however, does not guarantee the execution as the price may not reach the specified level at all.