A boiler is, more often than not, a part of a larger system that's used to produce heat. A boiler uses some fuel (whether it's natural gas, oil, or wood pellets) to heat water. The heat from the burning fuel is transferred through a heat exchanger, and into the water. Depending on the purpose of the boiler, it may be used simply to heat the water, or to increase the temperature to the point that the water becomes steam. Which sort of fuel, and what temperature the water is heated to, depends on the need of the commercial industry, and the type of boiler used.
One of two basic commercial boiler setups is called the firetube. This type of boiler may also be referred to as a shell boiler. Within this type of boiler there are tubes set up to be surrounded by water. The heat, fire, or gases are pumped through these tubes, and they heat the water which surrounds them. This is the most common boiler used in residential areas, but its commercial applications are the same. A firetube type of boiler can produce up to 25,000 pounds per hour of steam, which is quite a respectable amount.
The other type of boiler commonly in use is called the watertube. As the name might suggest, tubes filled with water pass through a watertube boiler, and the heat or gases are directed around the tubes to heat the water. Watertube boilers tend to be rather square or rectangular in shape with two or more drums attached to them. As the tubes disgorge their heated cargo into the top drum, steam continues to rise, whereas water and condensation drop down to the bottom drum. The water and condensation may then be used again to create more steam as the process continues on. This version is needed more when huge amounts of steam must be generated.