A bolometer is an instrument for measuring electromagnetic radiation by allowing the radiation to heat a metal whose electrical resistance is dependent upon its temperature. As the material heats, the change in resistance is noted.
Bolometers measure a relatively wide range of frequencies, thus are more suited to measuring energy over a wide range or estimating the total incident energy from a source.
Bolometers are not necessarily the most sensitive instruments at some frequencies. They can be made competitive in the submillimeter to 1 mm range (far infrared). To achieve sensitivity, such bolometers are run near absolute zero.
Micro-bolometers are used for imaging, with on the order of 100 x 100 pixels, and more recently approaching 1000 x 1000.
Note that the adjective bolometric which has the obvious meaning "having to do with bolometers" is very often used to imply the sum of all EMR, e.g., from a source. This is the use in terms like bolometric correction, bolometric luminosity, and bolometric magnitude. This came about because bolometers do indeed measure the power of the EMR striking it across a wide band, wider than many other instruments, though they are not actually perfect in that regard, and an Earth-bound bolometer will necessarily miss radiation not in atmospheric windows.