These units simply plug into the serial port (or USB in the case of the USB Units) of your PC (no power required) allowing you to measure temperature or humidity. The TH-03 uses precision resistive sensors, the TC-08 uses thermocouples, the PT-104 uses PT100 or PT1000 sensors, whilst the RH-02 has built in sensors for humidity and temperature.
They offer a very simple, accurate and cost effective way to monitor and record temperature and humidity on your PC using the supplied PicoLog software. For users who wish to write their own software, drivers and examples are included.
Interested in more technical information on thermocouples, go to this application note - Thermocouples