A high tech flame thrower to put it simply. It is most commonly referred to as a Heat Wave or Heat Ray.
The exothermic ray fires high quantities of infra-red radiation (heat) at a target. Usually at such high levels as to create a photo-thermal effect (ignite the air in its path giving the impression of a flame thrower). However unlike a simply flamethrower, the Exothermic Ray has the range of a beam and works be transferring the infrared to the target i.e. heat up what it hits.
This varies depending on the technology used and the design used in its construction.
While it's usefulness on organics is...obvious, most civilizations generally outlaw its direct use on other combatants.
An Exothermic Ray is best used against heavy machinery and Mechanoids, whose armor plating is impervious to most conventional forms of attack.