A market order is an order to buy or sell a specified number of shares (or bonds, etc. ) at the best available price when the order is submitted. All orders that don't bear a specific price are considered market orders.
Market orders placed while the markets are closed or before the market opens will be executed at an appropriate bid/ask price shortly after the market opens and contingent to trading volume on that particular stock. If the stock requested does not have sufficient volume to execute, then that order will stay pending until filled or canceled.