how do in floor heaters work?

I'm thinking about using hot water in the floor for heating an area. I understand how it works except the part where the used water goes?

You take hot water from the water heater, run it thru the floor in tube to transfer heat, then where does the water go?

Do they use a separate pump and heat the water with a separate heater/heat transfer so it's a closed system separate from your regular hot water system?