Stop losses are used to control investment risk and limit losses from downward price movements. There are two types of stop losses - protective, which is used for risky stocks in dull markets, and progressive, which is used for potential stocks in bull markets. Stop losses help prevent small losses from becoming larger by cutting losses at a predetermined price. However, stop losses also have limitations, as they can cut into potential profits and give false signals. When setting a stop loss, factors like one's risk tolerance, the stock's price habits, and market conditions should all be considered.