What is a good-til-canceled order and how is it different from a day order?

When you place a limit order, you choose how long it stays active. A day order expires at the end of the trading day if it hasn't filled, so if your price isn't reached the order simply vanishes and you'd re-enter it tomorrow. A good-til-canceled (GTC) order stays open across multiple days, often up to 60 or 90 days depending on the broker, until it either fills or you cancel it. GTC is handy if you want to buy a stock only if it dips to a target price and you don't want to re-enter the order daily. The risk is forgetting an old GTC order that suddenly fills on a price spike, so review your open orders periodically. Most routine index-fund investors never need either and just buy at market.