1- Limit Order: An order that has a specified price when it is posted for execution.

2- Market Order: An order that does not have a specified price when it is posted for execution. A Market order must be executed promptly at the best price obtainable, and will have priority over Limit Orders at the same price levels entered in the Pre-Opening period, and assumes an initial price limit value normally based on the most advantageous price on the market. A Market Order trades through a range of prices, starting with the best price on the market. The range of prices that a Market Order can go to may be limited by the Exchange.
Market Orders require a current market price, and therefore are not accepted until the market is loaded.

3- Day Order: The order is valid until the close of the trading day on which it was entered.