How can solar panels be used to heat water for a house or workplace?

Strictly, solar panels refer to photovoltaic (PV) panels, that turn sunlight into electricity. Heating water is a different system. Water is pumped to your roof where it lies exposed in some way to the sun, heating up. If the water in your ground level tank is colder than the water on the roof, the tiny pump keeps running, and your tank fills with hot water. There are many slightly different systems, but the principle is the same.