What's the difference between a leak detector and a shutoff valve?

A leak detector (sensor) only alerts you when it senses water; it can't stop the flow. A smart shutoff valve can automatically close your water supply when a leak, freeze risk, or abnormal flow is detected. Many systems pair the two: sensors trigger the valve. Note that some products in this category, like Moen's white leak-detector pucks, are sensors only and do not shut water off.