Cramer explained that when you buy a stock as a trade, you're buying it for a specific short-term catalyst, some anticipated future event that will drive the stock higher. This event may be a company's quarterly earnings, or it may news-driven, like a Food and Drug A drug approval. In either case, the plan is to buy a stock ahead of the catalyst and sell it afterwards. Once that window passes, however, investors must sell their trades, good or bad, Cramer said. An investment, on the other hand, is a long-term trading thesis that is not event-driven. Cramer said investments are not an excuse to buy and forget because they can still go wrong. That's why he recommends one hour of homework a week for each stock in your portfolio. As long as your investment thesis is still intact, he said, investors can continue to own their investments. "Never turn a trade into an investment," Cramer reminded viewers, even if that trade is a successful one.