A bolometer is a crystal coupled to a thermal sensor able to detect very small variations in temperatures. The bolometer is placed in a cryostate which allows to reach a very low temperature of about ten millikelvins. The principle of detection is the following : the ionizing incident particle deposits its energy in the bolometer under ionization, light and heat. Their proportion depends on the nature of that particle. The knowledge at least of two of these three quantities allows to identify the particle and then discriminate a nuclear recoil (induced by a WIMP) and an electron (induced by a gamma ray).
There are some crystals which provide two components, in particular Germanium which allows to detect both ionization and heat at very low temperature.
Information provided by: http://lyoinfo.in2p3.fr