It consists of an "absorber", which is connected to a heat sink (area of constant temperature) through an insulating link. The result is that any radiation absorbed by the absorber raises its temperature above that of the heat sink - the higher the power absorbed, the higher the temperature will be.
A thermometer of some kind, attached to the absorber, is used to measure the temperature, from which the absorbed power can be calculated. In some designs the thermometer is also the absorber; in others the absorber and thermometer are separate; this is known as "composite design".
While bolometers can be used to measure radiation of any frequency, for most wavelength ranges there are other methods of detection that are more sensitive. However, for sub-millimeter wavelengths (from around 200 μm to 1 mm wavelength), the bolometer is the most sensitive type of detector for any measurement over more than a very narrow wavelength range.
Bolometers are therefore used for astronomy at these wavelengths. However, to achieve the best sensitivity, they must be cooled down to a fraction of a degree above absolute zero (typically from 50 millikelvin to 300 millikelvin); this makes their operation technically somewhat challenging.