How does a coal based thermal plant work?

Coal is used to heat a boiler. The boiler heats water to steam and is then sent through a turbine. The turbine spins a shaft and produces power. The lower quality steam then goes through a condenser to turn it back into water, then the process repeats.

This is a simplified explanation of the process known as the Rankine cycle. Modern power plants have many little extra steps to squeeze as much power as they can out of the steam, but what I described (simple Rankine) is basically it.

A Google search will return a lot more detailed information if you want it.