A good for day market order is a type of market order that only last for the trading session in which it is opened. If it is not filled by market close it expires and is not active for the following trading session. In contrast, a good until cancelled order remains open until it is filled. In this respect both are actually limit orders, even though they are filled at market price when they become active. This type of order is often used by day traders, or by those looking to trade some news release.