A market order is an order to buy or sell a security (or broadly, an investment) at the best price available at the time it is placed. This order goes to the market signifying that the trader is instructed to buy or sell at the market price and is not much concerned with the price. A market buy order will fill on the ask or offer price while a market sell order will fill on the bid price.
On the other hand, a marketable limit order is a buy or sell order that the broker can execute immediately once it is submitted by a buyer or seller. Marketable limit orders are similar to market orders in almost all features except that they limit the price concessions that the brokers can make to fill them. Marketable limit orders with very high limit buy prices or very low limit sell prices are virtually market orders.
Marketable limit orders are used instead of market orders to limit execution price uncertainty and to limit their cost of liquidity. Both a marketable limit order and a market order execute immediately. However, a marketable limit order protects (if a buy order) the participant from buying at a higher price in case the offer price rises while the buy order is being submitted. It also protects (if a sell order) the participant from selling at a lower price in case the bid price falls while the sell order is being posted. This type of protection is not provided by a market order.