In most boilers, wood, coal, oil or natural gas is burned in a firebox to make heat. Running through the fire box and above that hot fire are a series of pipes with water running through them. The heat energy is conducted into the metal pipes, heating the water in the pipes until it boils into steam. Water boils into steam at 212 degrees Fahrenheit or 100 degrees Celsius.
The top picture on the right is of a small power plant located at Michigan State University. The black area to the left of the power plant is coal, the energy source that is burned to heat the water in the boilers of this plant.