Candy thermometers are, as you may have guessed, mostly used to measure temperature of a sugar solution whilst cooking. As sugar is heated it goes through stages and a candy thermometer can help you identify which stage of the process is happening, based upon the temperature.
It is well known that when you are making candy, you must stay within a certain temperature range. A candy thermometer helps you do just that, especially if this is your first time making some. The typical temperature range is from 220 degree F to 360 degree F. A quality candy thermometer comes with different ranges for specific stages of the candy making like soft crack and hard crack stage or the soft ball and hard ball stages. This is a scientifically based calculation and meaning you must pay attention when preparing candy because you can easily over- or under-heat the candy. This will destroy the whole recipe and wash your effort and expenses down the drain.
A meat thermometer is a thermometer used to measure the internal temperature of meat, especially roasts and steaks, and other cooked foods. The degree of "doneness" of meat or bread correlates closely with the internal temperature, so that a thermometer reading indicates when it is cooked as desired. When cooking, food should always be cooked so that the interior reaches a temperature sufficient, that in the case of meat is enough to kill pathogens that may cause foodborne illness or, in the case of bread, that is done baking the thermometer helps to ensure this.