Generally these devices refers to a jacketed glass vessel where a circulating chiller unit pumps a thermal control fluid through the jacket to accurately control the temperature of the vessel contents.
Additional to this, it is common to have a series of sensors (temperature, pH, pressure) measuring and recording parameters about the reactor contents.
These first systems were often home built and used hardware that was adapted rather than designed for the task Modern CLR systems take a wide range of forms with the ability to work on a range of different volume reactors (and indeed reactor styles).
In the most sophisticated systems that exist, analytical instruments such as raman spectrometers and FTIR probes can also be integrated with the reactor.
These more sophisticated systems also allow the closed loop control of the reactor as a result of taking readings from the sensors and analytical instruments concerned.