A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil.
The heated or vaporized fluid exits the boiler for use in various processes or heating applications,
including water heating, central heating, boiler-based power generation, cooking, and sanitation.

There are two basic types of boilers: firetube and watertube. The  fundamental difference between these is which side of the boiler tubes contain the combustion gases or the boiler water/steam.

In firetube boilers, the combustion gases pass inside boiler tubes, and heat is transferred to water
between the tubes and the outer shell. In watertube boilers, boiler water passes through the tubes
while the exhaust gases remain in the shell side, passing over the tube surfaces.

Because tubes can typically withstand higher internal pressure than the large chamber shell in a
firetube, watertube boilers are used where high steam pressures (3,000 psi, sometimes higher) are
required. Although firetube boilers account for the majority of boiler sales in terms of units, watertube
boilers account for the majority of boiler capacity.

The State of Michigan requires all commercial and industrial settings to follow the rules for installing,
maintaining and testing Control Safety Devices (CSD) on automatically fired boilers. CSD-1 is a standard
that is part of the American Society of Mechanical Engineers (ASME).

The State of Michigan rule R408.4027 adopts the CSD-1 standard and requires devices be tested
annually by an individual with a valid mechanical contractor license.

References:
http://mirules.elaws.us/rule/r408.4027