There are three main types of meat thermometers on the market today: Digital instant-read, dial, and Thermocouples. These thermometers are differentiated by how quickly and accurately they read a temperature, and where the temperature gauge sits in the stem.
Dial thermometers are the oldest version of meat thermometers, the slowest and least accurate. The stem is usually thick and needs to be inserted up to two inches into the meat so the temperature sensor can take an accurate reading. They also can take a minute or two to get an accurate reading.
Digital Instant-Read thermometers usually have a slender stem with the temperature sensor only a ½ inch past the tip of the probe. They have a much faster response time than dials, but still can be slow to reach an accurate temperature.
Thermocouples are the gold standard of fast, accurate and dependable temperature readings. The temperature sensor is located at the tip of a very narrow stem, reducing damage done to the meat by checking its temperature and allowing you full control of where you are getting your reading. They are also fast, near instantaneous, and certain probes (like the ChefsTemp Quad Xpro) can be left in during cooking.