You will need a candy thermometer to make marshmallows or marshmallow fluff. I like the digital thermometers because they are easy to read. If you buy one, be sure it has the ability to warning beeper that can tell you when the correct temperature has been reached. This is very handy in case you become distracted.
Another variety of candy thermometer, and probably the most familiar, looks like a long glass tube with a thermometer inside. It has a clip on it to attach the thermometer to the pan. It’s more difficult to read than a digital thermometer, but it is much less expensive. There are also instant-read varieties designed for candy making. You should use what type you like and can afford. Not all thermometers register to a high enough temperature for cooking sugar syrups, and that's why you need a candy thermometer. So check that the thermometer you will be using registers a temperature of up to at least 175 degrees, or you will not be able to measure the sugar syrup for all of the marshmallow recipes. The right kind of thermometer will say it is for candy making.