A boiler is a vessel in which water is heated generating hot water or steam. They are fired or powered by one of the following:

Fossil-Fired Burner (Oil, LPG, Natural Gas): 65% to 94% Efficiency

Immersion Electric Heating Elements: 85% to 99% Efficiency

Electrodes: 99% to 99.5% Efficiency

These boilers provide hot water, steam, and electricity to various process & heating applications.
For fossil fired boilers, they come in 2 general types: Fire Tube and Water Tube.
Common boiler auxiliary equipment includes Deaerators, Feedwater Tanks, Condensate Tanks, Blowdown Tanks and Blowdown Separators.