A market order is an order type that executes immediately against the best price available. As long as there is liquidity (sellers and buyers), market orders are filled. A market order to sell will match against the best available bids on the order book, and a market order to buy will match against the best available asks on the order book.
Market orders are often used when a trader values a fast execution more than an execution at a specific price.. When placing this order, a trader values an immediate fill at the current best price available.
In a volatile market, a market order may receive an execution price significantly different from the price of the asset quoted when the order was entered.
Since market orders are executed as promptly as possible, it is generally not possible to cancel a market order after you place it.
On The PIT, market orders do not rest on the book and are treated as Immediate or Cancel. That means that, after the order matches against the book,any remaining size will be canceled back. This can protect a large order from ripping through the book.