What's the actual difference between a flood watch and a flood warning?

With flood watches trending for several states, what's the real difference between a 'watch' and a 'warning' — and which one means you need to act now?

A flood watch means conditions are favorable for flooding to develop — it's a heads-up to stay alert and be prepared, not a sign that flooding is happening or imminent. A flood warning is more serious: it means flooding is either occurring or is imminent based on current data, and it's the point at which you should be taking action, not just monitoring. This same watch-versus-warning distinction applies across most National Weather Service alert types (tornadoes, severe thunderstorms, etc.) — as a general rule, "watch" means the ingredients are there and you should be ready, while "warning" means it's happening or about to happen and you should act. For flooding specifically, a step up from a flood warning is a flash flood warning, which indicates flooding that will develop very quickly (often within hours or even minutes) and requires immediate action — this is the one where "Turn Around, Don't Drown" applies most urgently, since flash flooding is responsible for more flood-related deaths than slower-developing river flooding.