Boilers produce steam using heat from fuel combustion. Steam is used for mechanical work, heating buildings, and industrial processes. Boilers have a shell, setting, grate, firebox, water space, and mounted components. Boilers are classified by what flows inside - water tube or fire tube. Water tube boilers have quicker steam generation and higher pressures/capacities, while fire tube boilers are cheaper with easier steam demand fluctuations. Locomotive boilers are compact and self-supported, generating high steam rates for trains. Boiler performance is measured by evaporative capacity and efficiency.