A chocolate thermometer is used when melting chocolate (or “tempering” it, to use the technical term.) The temperature range on the thermometers has to start quite low; some start at 0 C (32 F), some start at 4 C (40 F).
You can get digital or glass tube ones, but all are usually about 15 cm long (6 inches.)
The biggest problem people seem to have with the glass tube ones is the human factor: allowing them to roll off, or inadvertently knocking them off counters, whereupon with admirable reliability they shatter on the floor.
Infra-red thermometers can be used, but because infra-red can only measure the temperature of the surface, you must first before taking a measurement stir the chocolate well to even the heat out so that you will get an accurate reading. To be fair, though, whatever thermometer you are using, you should stir the chocolate first anyway before measuring the temperature.
A thermometer (except for an infra-red one) should be put at least 5 cm (2 inches) into the chocolate.